It's not cumbersome at all, they have POS systems that track individuals at tables, not whole tickets. The server inputs for the table as well as which chair they are sitting in. Then it is very easy to split a ticket in a variety of ways. I think you are complicating the process in your mind or you frequent places that don't use technology.
I think it varies, that kind of tech would only benefit a regular sit-down restaurants where the chairs are in fixed spots and constant. It doesn't work for a busy bar with me shouting "3 more beers" as the waiter walks by.
I assume the same or similar POS tech is used everywhere but in most places here I assume waiting staff either ignore it or charge everything to "chair 1" at the table, simply because so very very few ever split a bill to who ordered it. Since no one does, it's less work taking the hit when someone does (and asking people manually) rather than assigning orders to chairs all those times when no one splits. Obviously, that would also change if more people would split - so I'm guessing that with tech for splitting bills without staff help so prevalent here, it won't be common to have staff split bills.
So yes, the staff can do it. But I don’t mind summing up my stuff on the receipt and just sending money to whoever paid, and the reason I prefer it is because it’s so much quicker when paying (the hassle takes place at a different time and place).
It’s not uncommon to be served by 2 or 3 different people who of course remember who orders an extra beer when they go get it - but that information isn’t recorded, it’s just registered to the receipt on the table. When the bill is brought to the table I think it’s more common than not that the person bringing it did not serve everything on the receipt so would not be able to split the bill without input from the customers.