1. Did most buildings have basements?

2. What were they like? Rubble walls, cinderblock, brick?

3. What about the floor? Slab concrete? Dirt?

from what I know, most buildings had a basement

buildings were made of brick/concrete
often with a concrete face on the bricks, ea the greek colums where often made using a heavy I iron standing , with bricks and concret around, to make it look like stoneblocks

about floors or ceilings I know little, I presume they where wood beams/planks in older buildings, concrete on supports in "younger" buildings.

my 2cent

As with many german cities it depends where you are. Official buildings and some highrises where either concrete or steel frame and brick, the latter often with plaster cover.

Elder houses are typically red brick again often with a plaster cover to appear on first look like concrete.

Cellars are very common and typically have a packed concrete floor and often a cycling from the same substance. Higher floors vary between

Timber
Timber supports with un-reinvorced concrete cover/filling
Concrete

With the first and last being more common IIRC.

Cinderblock is common post WWII and the hate object of people living in 1950s/60s build houses since you basically can not put anything on a wall.

For "elegant" and better business houses a mix of brick and natural stone was used with the later found at the edges. In that case the brick was typically plastered over

Thanks for the info!!