The unlocked phones I've used can be put into auto mode (strongest signal, I guess?) or preference mode. Carrier locked phones do not switch to other providers

The unlocked phones I've used can be put into auto mode (strongest signal, I guess?) or preference mode. Carrier locked phones do not switch to other providers unless there's no service. Then it'll switch over to another carrier but only allow emergency calls (911 in NA). I'm not sure what type of handshakes/communications happen in the background