@xianc78 @tadano The only problem is that you can only use Source as long as valve licenses it to you. In the older days of Valve, the engine was more popular and many devs could obtain a license (even Respawn, who would heavily modify it like how the same guys at IW modified Quake 3 into their own engine).
Part of the problem is that as this Reddit post shows, Valve's licensing department seems to be MIA and users there seem to suggest the best way to obtain a license is to make mods for it first or have "something to show":
https://old.reddit.com/r/SourceEngine/comments/1c68m6j/is_there_even_anyone_left_at_valves_licenseing/Another post says why: the main guy at Valve in charge of this passed away years ago and so now it's a matter of "I'll ask X about it":
https://old.reddit.com/r/SourceEngine/comments/8mssei/comment/dzq3774/This is as opposed to modern engines which have very clear terms for small devs, like UE5 and Unity (which you can see why they're the engines of choice) or FOSS stuff like Godot or Monogame (more of a framework tbh).
Also good luck getting a UE2/3 license, I know it got weird when Epic discontinued UDK downloads for that even and they really want people to use the latest UE now. I've heard that there's license encumberment with those engines possibly too due to the use of third party components.
ID Tech 4? ID Software could get shut down by Bethesda/MSFT tomorrow and you could still use it as long as you respect the GPL (so no console releases ever, but consoles are dying). It's the same shit with the Torque engine, where GarageGames is defunct and yet commercial games still use it because of the MIT license.
Until the FOSS release of ID Tech 4 and people coming up with alternatives to the encumbered models (there's someone on moddb trying to make public domain assets for the base engine to replace this btw), the only way to really make a standalone game was you'd buy a license.
This is why Prey and Quake 4 look like Doom 3 in a way but also have their own distinct art direction, especially as both games revolved around indoor corridors with some outdoor areas. As time went on, devs invested in the engine made heavy engine modifications leading to games like ET: QW, Wolfenstein 2k9, and Brink looking nothing like Doom 3.
Much of the engine "look" has to do with the tech of it, and the ID Tech 4 engine excelled at dark indoor/night environments with lighting casting distinct shadows, like how some UE2 games had that "look" (Bioshock, Deus Ex: IW, Splinter Cell, and more).