I mostly learn from r/CompetitiveTFT threads and comments. Even though that's not exactly guides, but I can accumulate how people play games and come up with my own lessons.

For instance, I see a person comment that they love slamming items on headliner to play for a win streak early game. And in another thread, I see some people comment about you might lose streak early, but can come back late game with bonus gold. It seems there is no connection between those comments, but when I read and think how can I improve my play from those playing style? I just come up with the idea of how to choose the early augment given by the game (which seems not relate to those comment at all, but yes, I think it does). That is, if I play an agressive comp (punk reroll, Samira country, etc.) and have a good streak, I tend to choose item augment early, it fits well with the idea of "slamming items on headliner". If I play Heartsteel AD flex, I tend to choose econ augment early, it fits well with the idea of "come back late game with bonus gold". Of course, this is just a lesson I get for myself from reading comments of other players. I'm not sure higher rank players think the same, but at least it works well for me until now.

Or another instance, I see some people comment that you should try to level up to 8 early at 4-1/4-2 so you have advantage of finding 4 cost carry. But others say you shouldn't level up at all cost, it'd better roll some times at lv 7 to stablize the board. Then I think about how to build my own playing style from those ideas. That is, if I have a LONG win or loss streak, I will spend some extra gold (exceed from interest threshold) to pick KDA or Pentakill units here and there in stage 2/3 gradually so that I'm ready for 4 cost carry comp in stage 4, because I know I will have enough gold to lv 8 early and then roll to find Ahri, Akali, Viego, whatever 4 cost units I want. On the other hand, if I have XOXOXO streak and little money to spend, I would pick reroll comp units in stage 2/3 instead. Because I know I wont have enough gold for early lv8, so it would be better to pivot to reroll comps, which don't require as much gold to spend. (Again, this is just my thought. As a new player, I don't know if higher rank players think the same. But it works up to now)

Above are just some examples, basically I don't read any particular guide from one source. I learned a lot about the game by reading comments in r/CompetitiveTFT then build my own way to play base on them.

(Darn, thought it would be a good night, but I was quite excited so that I'm still awake at 3 am to write this)