SOLVED, with a huge thank you to !Donovar@lemmy.world
https://lemmy.world/comment/6271130

Hiyo Librecalc pros. Hoping someone has an idea how to automate or minimize the steps for this. Or where should I start? I'm trying to automate transcribing a list from:
Closet_01
Cartridge123
Cartridge234
Garagenook_01
Cartridge456
Toolbox567
Toolbox789
Garagenook_02
Cartridge890

Into an array/table like:
Cartridge123, Closet_01
Cartridge234, Closet_01
Cartridge456, Garagenook_01
Toolbox567, Garagenook_01
Toolbox789, Garagenook_01
Cartridge890, Garagenook_02

I get it; I should just carry around a barcode list of all my shelf names and take turns between scanning the list, tabbing over on my phone, scanning a box, then tabbing back again. But... what if I want to be lazier and just scan things in an order and automate the rest later?

Is there a faster way than copy-pasting my way down?

I got as far as making an adjacent column with a formula to check for "_", then copying the location if "_" was found.