A lot of us are in tech and we come across these problems and just troubleshoot our way around them and don't really think about them, so I figured a "case study" might be helpful. ~~Image unrelated.~~
Anyway, yesterday I tried making an alt account on lemmy.world. When last we left the situation, I had been notified that my registration application had been denied, but I'd been asked to confirm my email and https://lemmy.world/u/sergio had been created. A couple helpful commenters suggested my account might still be under review.
- Today I tried logging in again. But when I entered my username and password, I got a popup that said "incorrect login credentials". Maybe I needed to verify my email again? I went to my email, clicked the "verify your email" link, and just got a page with text that said "verify email" and nothing else. Maybe it didn't like me using my username to log in? So I tried logging in with my EMAIL and my password, and still got that "incorrect login credentials".
- Maybe something's wrong with my password? I'm using a password manager but OK, let's just try changing my password. I clicked on the "forgot email" link on the login page. I entered my email address and clicked "reset password"... and nothing happens. No feedback, and I don't get an email. (10 minutes so far).
- maybe I should check https://lemmy.world/u/sergio ? Hey, that's no longer there, I just get an error! I check the user page of another lemmy.world user and it works, so it's not a server problem. Maybe my application really was denied? Was it really reviewed? What do I do now?
(my intuition is to try again with a different username, email address, computer, and browser from a different location at a different time of day...)

Yeah I've been on Lemmy for over a year and I have no idea how it works or how I even set it up in the first place. I use it on mobile and decided to try browsing it on my desktop but had no idea how to log in. What's my instance? Why does it seem like I have separate logins on multiple instances?Which user/password do I use?
Oh, and trying to submit things on mobile is impossible as well. I don't have permission to submit to some community because it's on another instance? I can't just, ya know, federate my existing login session? Well fuck me for trying, I guess.
I gave up and just went back to old.reddit.com. Reddit on PC, Lemmy on mobile.
You need to access communities from the instance your account is on. Your instance is https://lemmy.one/ - < this is the website you need to use to access Lemmy. If you wish to use another instance, e.g https://lemm.ee/ you'd need to make a new account there.
If you wanted to submit a post in this community for example, you'd navigate to https://lemmy.one/c/[email protected] - note the end of the url. community_name@instance_community_is_located_on
https://lemmy.one/ , same credentials as mobile
You don't, there's a pinned post on [email protected] to help you about this as well as your other questions