Even that isn't a clear bright line. For example, the To: header also comes from the MUA, and it's crucial to the message's transit. (In fact, the To: header is very much analogous to the stuff written on the outside of an envelope.)
To: actually isn't used in routing. MTAs and MDAs route messages based on the target address that's communicated in the 'RCPT TO' command, ignoring whatever may be in the To/CC[/BCC] lines, should the MUA happen to generate those headers.
The set of metadata used in routing, aka the 'envelope', also contains a return path, which is (normally) used for things like submitting delivery failure reports.
No, the To: and From: fields you see in your MUA is the software equivalent of my saying over the phone, "Hey, this is Steve - is Dave there?" In my fairly educated opinion, of course.
