If you ever had the issue of faces not facing the right orientation once importing them to roblox heres is your tutorial:
Select all your meshes with A+A in edit mode.
Click there, and search for the button FACE ORIENTATION.
Like in the image above, do you see all those red faces? they are the ones causing the issue, they are looking inside the mesh, not outside.
Once all your mesh is selected press SPACE and write RECALCULATE NORMALS,
Press that button, now your mesh should look all BLUE.
Now you know all yours faces are facing out, and you are ready to import!
If for some reason recalculating your normals automatically doesnt work, select each face that has the red colors and press SPACE and Write FLIP NORMALS.