[2] https://www.reddit.com/r/Zettelkasten
Personally, I save all my interesting links to a plain text file. If I want to process something I learned, I write an "essay" about it. I end up publishing ~15% of these essays to my personal website, so that others can benefit and critique my understanding.
However, there is no system I was able to consistently stick with and most of the time I don't want to think about where to put something, I just want to put it somewhere. I have come full circle now and just use a DIN A6 notebook where I draw a line for each day with the current date under which I jot down whatever comes to mind that day. It's got page numbers, so whenever I want to reference something on another page I can just make a note of the page number.
Additionally, I used to write into a inbox file in Obsidian but noticed that it got quite slow especially on my phone, so I went back to Apple Notes.
Notes:
I toss my notes in a shoebox and stick the shoebox in a cupboard. [1]
Eventually, I stumble on the shoebox, look at the notes and throw them out.
Ideas:
"Idea" is a four letter word.
I start on the thing I am thinking about or I let it go.
Ideas that I am not acting on aren't worth holding onto.
Knowledge:
I remember where to look should it be useful.
Or I Google it again.
---
None of this was true when I was younger.
Back then, I behaved as though my notes and papers would go to an archive in a prestigious museum. As though ideas were valuable and rare. As if there was a multiple choice test on the-material on Tuesday, the 25th.
I made these things ends in themselves.
Now I just make stuff instead of taking notes.
This makes it clear whether or not an intellectual interest is an entertaining rabbit hole or something I can act upon.
The hard part is to accept that there are many many interesting things that I can't participate in. Like compiler internals, space exploration, and DIY sawmills.
Good luck.
[1]: Boxes are better than notebooks and file folders. Each of which is better than bits.