For anyone who was using K-9 wondering why Thunderbird looks no different, it's because they aren't different.
They build both apps from the exact same codebase. Only difference between the two are the default color scheme, the branding icons, and the text strings of the application's name. It's literally just a choice of which brand skin you prefer.
Which, honestly, kinda cool. A virtually zero-cost way to keep a few K-9 stans happy.
No, autoconfig is a way for mail servers to tell clients the config they should use (ports, ssl/starttls, username, name/domain of the server, authentication for imap, pop and smtp). There are multiple standards by different mail client devs, eg. Thunderbird, K-9 Mail and some other check for autoconfig.<mailserverdomain>/mail/config-v1.1.xml and outlook etc. check for a "autodiscover" SRV record, or use autodiscover.<emaildomain> or check <emaildomain>/autodiscover/autodiscover.xml. As you see, it's a mess. Well, only Outlooks "standard" is, as usually you wouldn't care because M$ Winshit Crasher Server does that automatically, so they don't care about good docs. Thunderbirds autoconfig is pretty well made and documented tho.
Literally running into this problem. I have a microsoft account with shared calendar and just can't seem to add it to the outlook app. It keeps trying to go to my domain. There's a button to specify the provider, outlook, but it just won't do it. Can't tell what this closed source garbage is doing wrong, it shows the manual entry fields with my server's info entered wrongfully.