This question was asked (and answered) around 2 years ago, but not long after Roblox released the packages BETA which completely changed how you would accomplish this.
The instances I’d like synced are somewhat scattered around the game in different places (for example: a few modules in ReplicatedStorage, some various server scripts/modules in ServerScriptService). One solution could be to group everything up and create some packages. I’m not all against having some code used in one Roblox place and not in another for a given script in the package, but if I wanted to put some extra code in one of the scripts belonging to the package, this case would become a hassle to deal with.
So, what’s the best way to manage a multi-place codebase nowadays?