I understand that Tor cells are 512 bytes each, and that they are encrypted by TLS, and that they pass through a TCP connection from the client through Tor to the server.
But how do packets work one level above that? Let's say Tor Browser (Firefox) makes an HTTP GET request to stackexchange.com. How does it go from this HTTP request to individual Tor cells?
Is the request simply broken down into these cells or does something more complex occur?
Moreover, who initiates the multiple TCP connections through the Tor network?