GZDoom for Doom and many other games running on the same engine. Source: over 1 year ago

For modern source port check GZDoom https://zdoom.org/index or Chocolate Doom (for higher framerates and stuff) - https://www.chocolate-doom.org/wiki/index.php/Chocolate_Doom. Source: over 2 years ago

Also, if you want to make a shooter like those, you don't really want the original Build engine. What you want is a modern open source implementation of those engines. For Duke-like games you can try https://www.eduke32.com/ (Ion Fury was made on this) , for Doom try https://zdoom.org/index or https://dengine.net/. Source: over 2 years ago

I don't know, but if you want a retro game you might as well use a retro-engine. Ion Fury (2019) used the EDuke32 engine and there are also modern versions of the original Doom engine, like GZDoom - with modern games like Brutal Doom or SIGIL (2019). Source: over 2 years ago

Because zDoom and GTA anniversary releases are a thing. Source: almost 3 years ago

Alternatively, for step 1 just download the client from the official website. Source: about 1 year ago

Your outlook on DCS has never seemed to be very positive... Might I offer you a different experience that might suit you better? Here ya go. Source: almost 2 years ago