Q: Confused with thunderbolt to connect mbpro with iMac

Confused to connect macbook pro 15 (mid12) as an external display with iMac 4K (late 15) via thunderbolt... when cable connected nothing happens, cmd+f2 don't work.. a bit strange. I've honestly speaking expected that computers will automatically recognize thunderbolt connection and change display modes, pair each other as it happens on how-to youtube videos related to this issue, but nothing happens.. ((

Any ideas guys?

Thank you in advance