What does it mean when a texture file contains 30 individual textures?

Some texture files in City Sample such as T_Brick_PackedB contain 30 individual 4k textures when exported.

(1) Is there a name for that kind of texture file?

(2) How can we identify that kind of file? The only way I see is by file size and that they have extreme resolutions like 40,000x40,000.

(3) How could we reduce those 30 files to 2k and then reimport them into a single file again?