Also for those interested in the Users: "There is Nothing", and "The word of the day" are both pretty important. You can type in the different Users in the user name, in "There is Nothing", and get different responses, while User2 talks to you in "The word of the day".
However, The Happy Event "Trinity p3 Non est deus" is perhaps the most important for understanding them all.
User3 actually shows up in that Happy Event to apparently save you. User 1 is the one in control of you during it, and User2 is trying to access you, but can't because you can't give it permission (you'll need audio).
User3 also notably shows up in Yumi's "Abyss" event, and Sana's "Ad infinitum" event where it says it's name is Pareidolia.
There's probably a lot more, but most of this is just based off memory and I haven't replayed the game in a bit.. so, sorry.
Overall, most disturbing stuff seems to be because of User1 who is likely Nozomu, the callous god, and strongest in summer, the computer-ish stuff is User2 who is likely the god of wires, the calm god, and strongest in winter, while User3 doesn't show up much unless needed to bring order, is Pareidolia, the concerned god, talks in lowercase, and has no season it's strongest in. User4 is reserved for someone/something and that's about all that's been said about it. There doesn't seem to be other users.
Oh, and Terminal 23 is likely Sensei or at least his mind.