Hey, thanks for trying it!
Not random. Customers only order from the 4 dishes you put on the menu, what changes is which courses each one goes for. Leveling up a dish gets you happier guests and better tips, and pricier dishes mean a bigger bill.
Also, from level 2 some regulars start showing up and each one has a favourite dish. If you have it on the menu they'll pretty much always order it, and they bring you gifts once they like you enough.
But yeah, the game doesn't really tell you any of this at the start.
I'll try to make it more obvious in the next update.

