It’s been a hot minute since this drama aired but I do remember being satisfied enough with the fantasy lore and explanations they provided by the end. Thought it was a bit complicated to follow.
This page (along with comments on KA) has some good explanations https://mydramalist.com/discussions/immortal/68055-bulgasal-theories-spoilers?page=6
One thing you got to remember is that all three leads have different agendas at the beginning of the drama but none of them has memories of the full timeline. They each start off at a midpoint which gives them a distorted version of the truth, with unfortunately the most selfish one being closest to the truth, furthering misunderstandings.
The ML just wants some good old revenge for the murder of his past self + family from his earliest memories. The SML enjoys the supernatural power trip way too much to give it up, along with being obsessed with the main leads. Meanwhile the FL wants all this supernatural nonsense to end at any cost. All three of them hold some power of a Bulgasal. The SFL helps them figure their shit out with her ability to peak into the past.