How the main scenario is made (but it's not going to be much different for sidequests

What Yoshi-P's job involves (talks a lot about patch cycles and its contents, a lot of which is quests).

On Apr 5, 2019, 3 months before the release of Shadowbringers, from https://forum.square-enix.com/ffxiv/...=1#post4941071
Q: I’m very excited for the story in Shadowbringers. How much of the story has been written so far?
A: While I can't go into the details of the upcoming story, in terms of progress, we’re about 80-90% complete with the main scenario. Things that remain are things such as the sidequests and checking over the completed sections.
Implying sidequests weren't complete, if even done at all, that close to the expansion release.

This doesn't provide a lot of detail on how quests are made, but has some comments on making the MSQ by the writers

Comments on localization which include how much work an expansion MSQ is.