I'm one of the devs on a relatively big mod, we try to stick to one release per major mc version, we were running 1.21/1.21.1 but recently changed to 1.21.4 because our mod is for a server that the majority of users use so us being on an older version blocks it from updating and they asked us to update.
Some updates are easier than others, it really depends on what changed. I wasn't developing the mod when the 1.19 ports were done but I was for the 1.20.2 port which wasn't too bad but I personally did most of the 1.21.4 port with some help at the end from another dev.
It's probably not the best way to do it but I just upped the mc version, tried to compile and see what broke. Sometimes it's just an argument removed or added to a method, sometimes a class got changed to a record. Other times a system has been completely reworked and at that point I'd go to the Fabric post or look at the NeoForge primers for more details on how those systems changed. For example, in 1.21.2 entity rendering got completely redone so we no longer had access to the entity itself in rendering so needed to work out how to get access to that which was a bit of a pain.
But basically it's mostly dependent on when you have time, the resources are out there to make the actual process easier.