This assumes the game isn't designed from the beginning to be entirely reliant on the operation of servers. For instance there never has been and never will be any form of offline function for Destiny 1 or 2, even if all the person wants to do is play the single player campaigns.
Destiny 1 partially ran on consoles, though. It was mostly p2p: https://gist.github.com/nessus42/55c1260d88e2028689de0c49381...
Yes, but never in the absence of the Bungie servers. It did have a full sequential single player campaign, but parts of the single player campaign took place in shared instances where the user could see other players running around the map. The single player campaign could also be played cooperatively with other players in the same 'fireteam' if desired, but that was optional.
Sure, but SPC still took place in the same shared world. It wasn't like there is a single-player vs multiplayer in CoD or BF. The only way to make those campaigns work offline is to remove all drops from them and remove co-op pieces that aren't optional. Doing a strike and joining right at the end of it, missing the entire story of that strike, was like a mandatory experience for D2.
Do we know any of the technical details of that? Is the SPC just instanced multiplayer with all game logic on the server, or is it just getting some encryption key?
First, SPC, at least in Destiny 2, required you to do Strikes. Second, you get item drops. Technically there is no SPC in either Destiny because it's an MMORPG. There are just plenty of things you can do all by yourself and things that you meant to do alone, and those things usually come with item farming, so you need server-side to make it fair.