How do i make a floor mesh

hey lots of videos on how to make floors or walls or whatever

allllllllllllllllllllll of them skip over the fact that the “default standard optional files” floor mesh are 400x400
completely unexplained, loaded one up to clone it didnt see anything adjustable or modifiable at all

now i know i am likely going to have to use a custom important floor mesh
so im looking out for information on that too BUT i am not there yet and dont want to be

i literally just need to drag something into my blueprint because that ! has to be done
and there is nothing out there for how to replace the standard assets with your own
nothing about floor tiles etc

literally just trying to make a room nothing complicated

Geometry brushes https://docs.unrealengine.com/en-US/Engine/Actors/Brushes/index.html.