How to bring my walkable game floor into my model

Hello

Honestly said, this is the type of reply a newbie like myself finds rather not the slightest helpful.

When you look at my model, do you think I don’t know what an fbx, a convex shape is?
How do you think I built a 1.9GB model over two years and finally got it from SketchUP to Twinmotion to UE 5.4?

And yes I am at a loss for many exact terms and that makes finding the right tutorial cumbersome.
I am not in my 20s but rather a 55+ autodidact.

I don’t mind at all being shown that I am a beginner but when a reply does no contain any real answer to my question I wonder why give it? So things take more time to sink in.

I am not new to the world but only to UE. and yes I have joined an online course.

Since then I have managed to extend my floor into my model and yes I am learning collisions etc.

Anyway thank you for your time.