Browser Isolation Software

Maybe checkout an OS like Qubes, Tails.

That is built to use this concept, not only for browsers.
(Here is a comparison).

https://www.comparitech.com/blog/vpn-privacy/anonymity-focused-linux-distributions/
(link added)

1) I want to develop remote browser that works as proxy or works like RemoteApp.

I'm imagining you're talking about a Remote Desktop App.

Verify this is correct.

2) How can I build such software with open source ?

Describe exactly what functions you'll be doing on each remote machine.

3) Do they use Server Side Rendering ?

Unsure what this might mean, as this doesn't really relate to either of your first questions.

There is no server side work done at all with a remote app, as "remote app" means some app running on a remote machine.

Server side or rather where connection is initiated may not even display any output.

Just depends on answer to #2.

Thank you for comments.

I want to develop a software that works on Windows client.
Main system needs to work on Linux.

 A) Works like Windows RemoteApp, but server is Linux.
      Windows client software --> browser on Linux --> Internet

 B) Works as proxy server. Configure proxy setting on IE,Firefox or Chrome on
      Windows client and control browser on Linux remotely.
      In "Browser Isolation Software", Windows browser with proxy setting can
      access internet through browser on Linux server.
      Windows client browser with proxy setting  --> browser on Linux --> Internet
 
Windows user can access Internet through "Browser Isolation Software" on Linux.

Hope this will help advice

Describe the purpose of your system.

Because there are many existing system which already do what you're describing in various contexts.

Defining your context will likely mean you can use an existing system or modify code of an existing system to accomplish your outcome.

Specifically...

When you say... "Works like Windows RemoteApp, but server is Linux"... Provide an exact set of steps of browsing some Internet site + what you'll be trying to accomplish by browsing a site....

Like...

1) Checking site speed.

2) Checking browser speed.

3) Scraping data off a site.

4) Any other action.

You can use software like nomachine's NX or x2go to setup a service like similar to a "Windows Terminal Server"
The processes would run on the server, the display software (NXClient, Xsgo-Client) would run on the users system.

Take a look at...

https://www.sandboxie.com/

Or:  

https://techcommunity.microsoft.com/t5/Windows-Kernel-Internals/Windows-Sandbox/ba-p/301849

creating your own browser is a non-trivial exercise. From what I can gather these products act partially as a web proxy and a http remote desktop