I bought a 2,5” SSD for my thinkpad e595 and when I've opened it, theres something in the ssd slot. I bought it just with M.2 memory, so I don't know what
2nd HDD SSD Hard Drive Tray Replacement for T420 T430 T510 T520 T530 W510 W520 W530, Internal Laptop CD/DVD-ROM Optical Modular Bay Adapter to 2.5 Inch 12.7MM - Walmart.com
Lenovo ThinkPad T440 m.2 SSD and 2.5" SSD Install (how to + test run) - YouTube
1 Pc M.2 Ngff Ssd To For Lenovo Thinkpad X1 Carbon 20+6pin 26pin Ssd Adapter Board Card For Pc Accessories - Pc Hardware Cables & Adapters - AliExpress
BEWARE: Lenovo Thinkpad L15 Gen3 SSD is practically un-upgradeable. Laptop comes with 256GB and bigger SSD is too thick to fit. There also goes a cooling plate on top. : r/thinkpad
Add Additional M.2 2242 SSD to T480 : r/thinkpad
How to install an mSATA hard drive/SSD in a Thinkpad T420 - YouTube