If I am on a phone call and I mistakenly leave the incoming caller screen, can I put the call on speaker?

Yes, just go back to the Phone app and you will be able to. Your profile shows you have an iPhone 14 Pro, you can actually go back by tapping on the phone icon you should see in the dynamic island on the phone. That should also take you back to the phone call screen. From there you can tap the speaker button.