isn't bottom up learning (the typical approach in english learning books) for grammar kind of useless?

You forgot them because you didn't use them. There's still value in studying those sorts of books, but you need to actually use the material to retain it.

Personally, I think learning with grammar is very difficult and boring. I would suggest to naturally learn the language by listening to native speakers that speak in a clear and easy way Here are two examples! https://youtu.be/_BfvucYgteY?si=RjpKS6Tdl-C8Z5nB https://youtu.be/-1iaGaam5Q0?si=sy6YoZNkwWx4I5WD

You're stumbling upon another school of thought in language learning known as "comprehensible input" The gist of it is that you aren't going to acquire a language by simply studying grammar rules, but you can get a greater feel for them by simply consuming target language content that you understand 80-90% of (enough that you can fill in the extra 10% from context but not so easy that it's not teaching you anything) I won't say one is right or wrong but yeah, many people nowadays are coming to the realization that fluency won't come from studying workbooks

To answer your question, when I learnt English it was through reading fiction and by listening and interacting in English with people around me (I grew up in an English-speaking country). When I learnt Italian and German I went through the process of learning the grammar from books because they provide a good summary of the language. However, after a certain point, they became rather tedious so I started learning more by listening and watching videos. I was also taking lessons and trying to practice speaking with other learners as much as possible. I’m not learning intensively these days but I try to watch some youtube in Italian or German. I don’t even know where my grammar books are anymore, but they were certainly helpful at the beginning.

the entirety of my knowledge of English came from consuming media in the language, that being movies, shows, music and specially videogames, also a bit of talking online and reading stuff. in short: being exposed to the language. this typa teaching always felt terribly ineffective to me, also being extremely boring