This article/video shows the final 800 section of the USN John F Kennedy being assembled. Something similar for larger starships? While it makes sense for some to be built in orbit, you can imagine that rather than a crane a large section is assembled on planet, taken into orbit on a cargo lighter and then tugs use thrusters to place this section in place and weld it together.
https://www.dailymail.co.uk/news/article-7305355/Final-780-ton-piece-11-3-billion-aircraft-carrier-USS-John-F-Kennedy-place.html
https://www.dailymail.co.uk/news/article-7305355/Final-780-ton-piece-11-3-billion-aircraft-carrier-USS-John-F-Kennedy-place.html