(1) attitude -- being willing to make mistakes (it's actually part of the brain rewiring process -- we remember better when there's an emotional connection). Also being willing to take the language as it is, and not imposing your own values on it from the get-go (e.g. why are there so many unnecessary cases/conjugations? it's stupid! -- people who are super-contrarian and judgy probably struggle more... they try to analyze everything but end up not learning anything. It's a common attitudinal problem among many otherwise smart people.)
(2) constant contact -- you need to spend time with the language in small chunks (e.g. 1/2 hour a day) over a long period of time rather than spend a week studying then stopping for several months.
(3) ability to notice things/distinctions -- this is important because if you cannot differentiate sounds or details, it can be hard to get to a certain level. I believe this ability is at least partly innate; folks who have this ability in spades seem to be able to progress faster. (but I also believe folks who have a growth mindset and put in the work eventually get there)
With these elements, one can assemble a combination of techniques tailored to oneself (everybody learns differently). For me, watching YouTube creators explain local culture is what interests me. Podcasts work for some people, but interesting content doesn't always exist in my target language, so I'm perfectly happy to reject that as technique for acquiring certain languages. Taking classroom lessons actually works really well for me because I like structure, so I do it weekly. Some people like music for learning languages -- it doesn't really work for me because I don't get to learn intonation (music obscures intonation and vowel lengths).
Learning techniques have to necessarily be bespoke because we come at languages with different backgrounds and interests. But I believe the 3 elements above have to exist.
In my opinion -- this is only my opinion -- immersion by living in a country is overrated as a technique. It helps for sure, but it really isn't necessary. The elements above are what are needed -- without these, even immersion is ineffective. I've met Irish guys living in Barcelona who couldn't speak any of the local languages. I've first hand experience living in several places where I never learned the local language because there was always a way around it.