The two wires on that screw are:
- The end of the supply wire coming from the breaker panel (or an upstream location), stabbed into the left switch.
- A jumper taking power also over to the other switch, stabbed into the right switch and screwed into the left.
In this case, they use both the backstab and the screw. This is not good practice and is a super lazy installer trick. (frankly the same could be said of backstabs generally, they are not reliable in the long term.)
I recommend you pigtail both switches. The wires I mentioned, pull them both out of the backstabs. Now the left one has a pigtail, that was easy. Get 6" of #14 solid wire and put a similar pigtail on the right switch, use the screw not the stab. (backstabs can't be used twice.) Now use a wirenut to join both pigtails and the power supply switch.
Suddenly this all will make a lot more sense.
I am a huge believer in designing the junction box to make sense, rather than trying to remember a complex configuration, and then having to reverse engineer it later. My primary weapon is colored electrical tape, wrapped a few times around the wire to mark it. So I would mark the bottom wires on each switch, yellow for the lamp and blue for the fan.