There’s multiple selections:

[permalink]

• PRIMARY: is the old-school Unix one; select text to copy it. Middle-click to paste it. Three-finger tap on a touchpad also pastes it.
• CLIPBOARD: the well-known ctrl-c and ctrl-v (or Super+c).
• SECONDARY: historical, irrelevant, don’t worry about this one.

Vim (& derivates)

[permalink]

Vim has registers. They are accessed by prefixing "X, where X is a register name. These extend the ones above:

• "-: /dev/null
• "*: (see :h unnamed), the PRIMARY SELECTION
• "+: (see :h unnamedplus), the CLIPBOARD SELECTION
• There’s also a lot of other registers which are historical and exotic IMHO

Neovim config example:

[permalink]

Copying to the PRIMARY selection:

-- Copy to PRIMARY selection on mouse selection.
vim.keymap.set("v", "<LeftRelease>", '"*ygv')

-- Yank (copy) to PRIMARY selection by default (e.g.: with `yy`).
vim.opt.clipboard = "unnamed"

Copying to CLIPBOARD is also possible, but confusing in terminals:

-- Copy to CLIPBOARD selection with ctrl+c.
vim.keymap.set("v", "<C-c>", '"+ygv')

-- Paste from CLIPBOARD selection with ctrl+v.
vim.keymap.set("v", "<C-v>", '"+p')

Note that the usual mappings for Ctrl+Shift+c (or Cmd+c) should work fine on terminals.