That's leaping into spoiler territory so I'm gonna do my best to censor this. At the end of the game,

|Asgard is obliterated in ragnarok, thus unexplorable, though there are portions of it across the other realms to explore|

|Vanaheim, Svartalfheim,Helheim, Alfheim and Midgard| are the most widely explorable, especially post game when you have all your upgrades for traversal and puzzle solving

|Muspelheim| is explorable to an extent, it also |has trials like in 2018|

|Jotunheim| can be visited but there isn't much to do at all, it's just for visiting and such

I cant remember much about |Helheim and Niflheim|, as I haven't played in a couple months.