You can see all registers in use with :registers, to paste from a register say "2 in insert mode use key combination <ctrl-r>2 or in normal mode "2p. You can check out more in :help registers. Unnamed register or "" is the system clipboard I think. To copy texts in a register you can prepend yank (/delete/cut, etc.) with that register "_ (for black hole register[^black_hole]) This is for neovim. Have keybinds for them and there saved you a plugin :D

[^black_hole]: Text yanked in this register is gone, i.e. it's not saved in any register.