For making models, if you are restricted to online, I find Blockbench to be the best:
https://web.blockbench.net/
The generic model works for low-poly modeling and animations.
If you have a computer that can run it, I would recommend Blender, it is very powerful and free, but it takes a while to master.
For the multiplayer side, if you are following the glitch tutorial (glitch no longer works, but Replit works the same way) you can customize the local and other player models that are cloned, and you can use socket requests to change things about the characters such as animations or skin changes.
For guns, this is a good tutorial:
For multiplayer just do the same logic but clone the bullet on everyone’s side coming from the player that shot it.
Hope this helps!