trying to understand a string literal in a basic server program

[CompleteNewb]

I have this code:

import socket

HEADERSIZE = 10

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.bind((socket.gethostname(),1234))
s.listen(5)

while True:
    clientsocket, address = s.accept()
    print(f"Connection from {address} has been established!")

    msg = "Welcome to the server!"
    msg = f'{len(msg):<10{HEADERSIZE}}'+ msg

    clientsocket.send(bytes("msg", "utf-8"))

    while True:
        time.sleep(3)
        msg = f"The time is! {time.time()}"
        msg = f'{len(msg):<{HEADERSIZE}}' + msg
        clientsocket.send(bytes(msg, "utf-8"))

and i'm trying to understand this line: f{len(msg):<10{HEADERSIZE}}

so i opened my ide and i did this:

>>> msg = 'Hello World!'
>>> len(msg)
12
>>> len(msg):<10
SyntaxError: invalid syntax

for me it's pretty much the same thing so... why doen't it work?
Plus, if you remove the string literal in line fourteen, doesn't give : msg = 22:<10 10 + msg???

I tried to find an explanation on the web, but to no avail...
Could someone explain what's going on?
Could someone explain?