Copy on Select watches for text selections and pushes them to the clipboard, and (when clipboard sharing is on) to your other Macs, without pressing ⌘C.
Enable
Settings → Copy on Select → Copy on select (Pro; defaults on when licensed).
Skip editable text (on by default) leaves your clipboard alone when you select text you can edit: a note, an email draft, a message box.
Works alongside Clipboard sharing. Selections respect the same size cap and direction settings as manual copies. Copy on Select is a Pro feature; see Free vs Pro.
Detection
Detection only affects apps that hide their selection. When an app reports its selected text, every selection gesture copies whatever the level.
Precise never copies from apps that hide their selection. Balanced, the default, adds a drag or Shift-click, plus double-click and Shift-arrow on read-only content. Eager adds those everywhere, which may over-copy. Switch to Precise if copies fire too often.
Copied flash
Show copied flash (on by default) briefly shows Copied after an auto-copy. Turn it off if you would rather not see it.
Privacy
Concealed clipboard items, passwords and other secret pasteboard types, are never copied or sent. See Settings → Privacy & Security.