Hi, welcome!
In addition to extensions as mentioned above, I’ve found myself re-using some rules in several projects, but I haven’t yet bothered to pull out everything that’s common to them, because when I start a new project, I’m usually more focused on laying out some rooms, rules and NPCs specific to that scenario; whereas the common aspects are sometimes more for player convenience or for polish at a later stage. For example, making sure there’s a good default response for actions like SMELL for those objects which don’t have a specific response – that’s good to have in every project, but it’s not necessary to get started on a new game.
The role of a standard common base to jump-start the creation process is more or less filled by the automatically-included “Standard Rules”, which provide the material to let you hit the ground running (in this sense, I7 is more “batteries included” than some general-purpose game engines).
About source code, here’s another well-organized example (by Juhana Leinonen): Juhana Leinonen - Sparkle — 1 of 108
And there’s this useful post about organization: Structuring Inform 7 code | Undo Restart Restore
A list of games with available I7 source code: https://ifdb.org/search?searchfor=tag:I7+source+available
As for the general learning process, the manual “Writing with Inform” is a great place to start. Don’t forget to also look at the included Recipe Book, which organizes the examples by topic.
There’s also this big post: Inform 7 documentation and resources
(You don’t need to read all that now, just skim over it in order to know what’s there.)
Have fun, and don’t hesitate to ask questions in the forum. 