Many Browser options are available today – google chrome, mozilla firefox, safari, IE, maxthon, avont browser, lunascape, falshpeakslimbrowser, polypane.app (https://polypane.app/), Brave (https://brave.com)….
One can build their own browser, libwww is a library which includs all modules to create a Web application, such as http, html, etc. It has been initially written by Tim Berners-Lee, and was freely available. People could just take the library and put an interface on top of it to make a browser, a Web server, an indexing bot.
One of the very difficult and technical parts of a browser is the layout (or rendering) engine. It takes the Web content and displays it after having processed the style and scripting information. Some of these layout engines are open source such as WebKit, Gecko and KHTML. Others are sometimes sold to third parties for developing specific products. Web developers take them and create new browsers or use them in their applications.
Engines available – WebKit, Servo, NetSurf, Goanna, Gecko, EdgeHTML, Blink.