I am confused on game progression, can anyone eli5?

I am on my first character, a Shaman at level 45, and just been going through the campaign. I just got to the snowy burning forest looking for the spear.

Chat and Reddit both talk about only going so far in the campaign to get idol slots, and then swapping to monoliths. I have not unlocked all idol slots, and am in my mid 40's. But I see online people doing monoliths in their 20's?

I am further confused because when I went into what I thought were monoliths around lvl 25, everything was 50+ and destroyed me.

I am not sure what I am looking for or how to know when I have hit the correct points to transition between content? Is there any guideline or checkpoint to know?