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Android - Restricting device remotely

I've been looking into how to restrict access to certain apps, track location, restrict phone calls on an android device and have that dictated remotely. I've come across integrating an Enterprise Mobility Management (EMM) or Mobile Device Management (MDM) solution so I was just wondering if that is the only option here?

Well, Google has its own EMM called Android Enterprise, it offers some features as app restrictions, managed configurations, policies, etc. In backend you can catch all responses from device (ex. GPS). Sometimes if you need a specific feature you will depend from Google's team.
Also, some companies has a DPC app (Device Policy Controller), that app can control almost all feature in your device and send a lot of information to MDM or EMM, this app is configured as Device Owner. In this option, you have to develop MDM backend and DPC app as you need them.

Is EMM what I'm looking for?

We have a Java based web application and an android app for our enterprise needs. We would like to remotely push the android app from the web application and we don't know how to achieve this. After searching in Google, I found Android EMM but did not understand what that is exactly as there were no other blogs or tutorials other than the Google site which was difficult for me to understand.
It is similar to MDM solution but do MDM servers have an app preinstalled in the devices so that the installed app manages the installation of other apps?
Can anybody tell how to remotely push the android app from my web application?

To remotely push an app to an Android device you need to manage this device, you cannot just push an app on anyone's device. To manage a device you need to set up this device as managed, which will require a factory reset.
If you want to pursue this approach you can try the Android Management API.

You can just upload .apk file to your server and provide a link to the user. This is a very simple solution but requires user to agree "install apk from third party sources". In this case, a user needs to agree with installing apk.
If you want to install apk without user agreement you need to activate Device Owner on Android (Fred mentioned in the comment this approach).

Android-Server SMS Verification code

I have an android app and I want to allow a user to login using a verification code. I want to send the verification code in an SMS to the user's phone. What is the best solution to verify if the verification code matchs on the server side?
Is the creation of new table that contains verification codes a good idea?

Here are a couple ways to implement user verification on Android:
Some apps build it themselves with SMS / Voice APIs
Others use a verification specific API
With SMS / Voice APIs
You'll need to generate a code (huge security threat)
Send it via the channel you use primarily
Set some kind of time delay job (queue,worker, whatever)
Send a fallback message should you get no response
from the user
Nexmo’s Verify API
Instead of paying different fees for each country, Nexmo allows you to go globally with a simple API call for a flat fee of 10 cents per verification, NOT per attempt (unsuccessful attempts will not be charged towards your account)
Verification code should arrive to your phone as an SMS. If you wait a while, you'll also get a phone call, and the code will be read to you. The timing and channels used depends on the type of number, the country, and the carrier.
You can also use Verify's Android SDK, which enables you to build Verify into your Android app by simplifying this integration. After importing this library into your app, you will only need the user's phone number, while the SDK will take care of the remaining steps required to verify your users.
Take a look at Nexmo's Verify Android SDK here
Full disclosure, I work at Nexmo.

Remote sharing my app (Android/iOS) to a desktop user

I'm looking for guidance on how to achieve (build) the following:
I have a native App (supported on both Android and iOS)
I have a user 'A' who has the app installed on his phone.
I want another user 'B' to be able to remotely (on a browser) be able to "view" or "take control" of the app being used by user 'A'. So, User A could say "Share with User B" and User B starts seeing a virtual view of the app on User A's phone, and follows as User A navigates around the app.
Basically, some sort of screen sharing on phone, but only limited to my app.
Note that I do not want the user 'A' to install any separate app to share my app. It should be a capability existing/built within my app itself.

To achieve your goal there are smaller problems to take care about, from screen change detection to P2P communication between network nodes.
For iOS there's a Hierarchy Viewer github project that setups HTTP service on device, allowing to preview snapshots of app window using a web browser. Making use of that, you'd still need custom protocols to notify about user touch events and screen changes, which in turn requires effective screen change detector.
Note that this solution assumes no NAT stays between controlling and controlled devices, which is usually a case for local networks. To use the Internet as a medium, you should consider P2P protocols capable of NAT traversal (reffer to STUN protocol for endpoint address recognition and hole punching as a NAT traversal technique).

As a suggestion, how about this? Please note I'm not into this myself, so I might be suggesting something very hard to do.
Create a website that shows the same information as the phone
Make the app have a "server" component
Have the website authenticate and connect to the phone
Have the phone to send over the data from the app to the website, which can then show it in the appropriate fields/positions.

I think what you're looking for is some kind of VNC-like server that would be integrated into your application and serves the content of your application ui on the network. It should be technically feasible, yet it certainly won't be easy !
There are several VNC servers for android ; most but not all requires root access though, probably because they serve the phone screen and not one specific application.

Specific devices to gain access to page, but others cannot

I have a project in mind, but i was hoping i could get some insight.
Would it be possible to have a page allow access to my phone, or a co workers phone or laptop or device, but refuse any other device to gain access to this page.
The idea of this project is through QR codes, for example,
If i generate a QR code, to access a list of lets say, inventory. i would like my phone, or my co workers phone to gain access to it, but if say my brother or someone outside trys the code and gets sent to this specific page(s), it doesn't allow it to happen because the phone ID or tablet ID is not in the list to gain access.
It would be difficult to create a login page and enable QR codes, because you cannot implement the login information in the QR code because then the security would be irrelevant.
Any suggestions??

Your page could restrict access based on the originating IP address. The feasibility of such a system would depend on a number of parameters, such as whether you are connecting over wifi or cellular, in the case of wifi - whether you trust the wifi network, in the case of cellular - how often the cell provider changes your address.
A better solution might be to use a custom URI scheme and a custom app installed on only the phones and tablets that you want to have access. When the device scans the QR code, the app would handle the request, and then log in to the web site over HTTPS with proper authentication. e.g.
QR code: my.app://blah/blah
App handles URL, logs into server, redirects to https://my.site/blah/blah

Why not use identity certificates? Sign your message (QR Code) with the corresponding private key and validate for scanner's Identity, Trust and Message integrity.