air travel – I go round-trip from USA to China and back, friend only goes from China to USA

Two single tickets will almost always be more expensive than a return, unless you book a very flexible return which is going to be much more expensive that the cheap fares.

Actually, in some cases, it can be cheaper to buy a return ticket than a one-way ticket! Remember that you must use the first (“outbound”) flight even if you ditch the second one (“inbound”), you can’t skip the first flight or it is very likely they will cancel the rest of the flights on the ticket (so you can’t just book two returns and only use one of the outbound then the two inbounds).

To be on the same flight is easy, you can just select it when you book.

To have seats next to each other, the ideal option is to find airlines/fares that allow you to select your seat at booking time. Some airlines may allow it for all fares, some only for more expensive fares, some will charge a bit extra for the convenience, some don’t allow it at all, so you’ll need to do some shopping around. It may also depend on frequent flyer status. Note that in some cases you can do it if you book directly with the airline and not through a travel agent, and it may also be more complex if it’s a codeshare rather than the operating airline.

The next best option is to pick your seats at check-in time. Again, this varies a lot, with policies varying based on airline, fare and frequent flyer status. Check-in online as early as possible. If online check-in is not possible, then you need to be at the airport very early to be at the very start of the check-in process (but remember that even if it’s not possible for you, it may have been possible for others). Avoid dates where there are lots of people travelling (e.g. around holidays), the less people on board the higher the chances of getting seats together.

Another option, but that’s highly dependent on the airline, is to have them join the bookings. You’ll nearly certainly have to call them for that, and you should probably call them before booking to make sure this is even possible in this specific combination.