You can play many wads/mods without having to own the full version of Doom. Check your installation file for the 'wads' folder.
TIP: Most if not all of the modern Doom games (Doom 3 BFG / Doom Eternal) include the files needed for playing Doom 1 and 2. The Ultimate Doom can be purchased on Steam
To play any of the commercial games you will need to own them. This is just an engine port, no game assets are included whatsoever.
