The network explicitly tells a TCP sender how many unakcnowledged

bytes the sender can send over the network by setting the congestion window

inside the TCP header.

A TCP sender infers on its own how many unacknowledged bytes it can send

without overwhelming the receiver, based on the ACKs that the receiver sends.

A TCP receiver explicitly tells the sender how many unakcnowledged bytes the sender can send to the receiver, by setting the receiver window inside the TCP header.

A TCP sender infers on its own how many unacknowledged bytes it can send without causing network congestion, based on the ACKs it receives.