What application or services helps you learn the most?

I'm not sure what's best for Mandarin or Japanese specifically, but if you can read and want something that reduces friction, have you looked into LingQ? I am using it for reading, listening a lot in French. It also supports importing YouTube content.

I think at your level you should focus on native content... a lot of native content. Anything that helps you with consuming a huge amount of content is great. Use the language by consuming.

most apps like duolingo, bussuu, babbel usually teach you in a rigid path on topics you might not even have an interest in. For example if you are not interested in cooking, sports, the chapter on cooking,sports will be very boring.

Only apps I would recommend are those that teach in a high frequency way. ie teaches high-frequency words, sentences, grammar points and etc. I ditched apps as soon as I could read a book.

I feel apps like to keep you in a learner mode that is how they make money, I feel as soon as you can consume content in that language, you are a user of the language, so consume content instead. make mistakes, learn from them and keep consuming.

I like LingQ because I can import everything i want to consume in 1 place, from books, podcast, YT series, music and etc.

Imo, once you're at about the intermediate/upper intermediate, the best thing to do is probably try to find content you can consume. Anki and the like are incredibly useful, but the goal with those is to help you reach the point of decoding real messages in context.

It's hard because everything is difficult to understand. But I think reading is pretty approachable. In Spanish, I found a book of short horror stories aimed at probably 3-5 grade level, and it was pretty fun to read through, along with a couple other youth-targeted books.

For Japanese, I highly recommend starting with the free children's books at https://tadoku.org/japanese/en/free-books-en/ - they might be too easy, but they probably won't be too tough. They extend down to a nicely low level and have fun, cute illustrated stories. Satori Reader is a great app that has a bunch of short stories that you can read or listen to (paid sub, but you can read enough on free). There's also the NHK easy news page to try and read (or listen to).

For listening, there are of course a lot of great audio resources through Youtube or podcast feeds. I'll give one example for Japanese that I found especially approachable: these comprehensible input playlists from いろいろな日本語: Beginner, Intermediate.

But if you do want apps in particular, I think Renshuu and jpdb are pretty good. Both have Anki-ish spaced-repetition-system stuff as core components. Renshuu has lessons and games to checkout. jpdb integrates its SRS with a database of various media to help target your learning. There's also bunpro for grammar (paid sub for features though).

For something not Japanese specific, I think Memrise seems pretty cool lately. Now seems to have more videos and other content in there. I don't use it as much though, since it wants to play a lot of ads.

I found this FOSS app https://librelingo.app/