3d model levels?

Can you use 3d models to create a level? Like in any normal 3d game? Or is there a special program?

You can create levels using SRB2 Doom Builder, SRB2 Doom Builder 2, or SRB2 Workbench. 3D model rendering is only a feature of OpenGL and they don't support true collision. SRB2 is technically just a really good 2.5D engine; it's not actual 3D!

If you are interested in level editing, then try this link. It is a hub for all of the information you could possibly need. Start with Level Design 101 and SRB2 Doom Builder Tutorial. Some of these may not be updated for 2.1, so beware.

https://wiki.srb2.org/wiki/Level_design

SRB2 Uses Doom Engine,it has it's own Map Building Program called Doom Builder,SRB2's Doom Builder can be found here ! and CFG File to update it to work with SRB2 2.1 here !

I once made two levels which use 3d models in the stage, and made platforms in DooM Builder to be placed in the exact spots where the model is located, but otherwise, no, 3d models as maps is not possible.

Hypothetically you could make MD2s to use as scenery, or even as a replacement for visible level geometry by laying it on invisible stuff, but besides forcing it to be OGL only, you'd run into the problem where they stop rendering if the mobj they're from passes behind the camera.

So it's not really viable for critical stuff, but you can certainly use them to pretty up the place.