How to bring my walkable game floor into my model

Hello Thank you for your reply.
That is somewhat deflating my high energy
I have never done this before this is a first for me. I am NOT familiar with collisions yet
when I open the prop folder I see props that I do NOT have in my model
I guess they are from the UE original play platform maybe

Do I need to move my entire started content into props and delete the ones that aren’t in my model?
In the future there may be a limited number once I learn to create “collisions”
my space is dedicated to move around walking.
It is NOT intended as a shoot run and jump game rather a peaceful meeting space.
Now all I want it so simply walk around (even if I walk through walls at the beginning maybe
although unlikely as I can navigate open spaces.
I just was hoping for indication on how to expand the “game floor” rectangle in the foreground to extend into the building
Thank you