I was able to pick seats on the United mobile app for an upcoming united ticketed flight involving Swiss and LH (it's in C). I was also able to preorder a meal on the LH website. The United App provides a booking code you can use on the LH app/website. I found the website easier as they let you type in the flight number and month (use Flightaware for the LH flight if it's a United 8### type number) and you can bring up the menu that describes the meals as otherwise you just see pictures. They also have 3 bonus meals available for pre-order that aren't on the menu). I last flew one of these multi-carrier United code share tickets in 2022 and I have to say I'm quite impressed with the experience so far...I used to dread these code shares and the extra calls, but this is seeming to be pretty cool if everything stays as I selected.