The top side is designed for M.2 key B 2242 cards. 3042 are fine too, but are wider than the U101B.

• LTE or 5G cards
• dual nano SIM
• eSIM optional

The bottom side is designed for M.2 key E 2230/2242 cards.

• Wifi (note: USB 2.0 only)
• GNSS
• GNSS RTK
• LoraWAN
• M.2 cards are mounted with M3 screw.
• 4 mounting holes 2.2mm
• Size: 30mm x 60.7mm
• USB: USB-C with 5V and USB 2.0

It has an integrated 4 port USB 2.0 hub (only 2 ports used).
It has a Flat bottom. So there are no components on bottom side.
U102 supports double sided M.2 cards (bottom clearance: 2.5mm, but reduced by the components on the U102).