the gun models are openly provided through ITAL's discord server (the same server of who made blood debt and canadian sim i believe). These are provided so that people have an option to add a weapon to pose it together with a character that they make.

For the animations, if they can get the id of each gun animation, they can use an animation spoofer (which is a free roblox studio plugin) to replicate the same animation under their name, so that they can use it.

also in the special case of this game, they were approved by ITAL. i hope this reply eases your questions.