In which scenario does the client act as a server?

In active FTP mode, the client sends a PORT command to the server specifying a local port it is listening on, and the server then initiates the TCP data connection back to the client on that port. This reversal of the data channel means the client accepts an incoming connection from the server, fulfilling the role of a server for that data transfer.

SMTP is a client-initiated protocol where the sending host always connects outbound to port 25 on the mail server; the client never listens for or accepts incoming connections.

During an SMTP connection, the client pushes mail to the server by initiating the TCP connection; the client does not open a listening port for the server to connect back to.

When browsing websites, the client sends HTTP/HTTPS requests to the web server and only receives responses; all connections are outbound from the client.