Speaking for myself: working out, reading, composing music, and journaling.

Learning a language not spoken around you is very solitary experience.

Also, jogging and swimming, drawing, playing musical instrument.

Learning a language in this way is some sad boi stuff

Why? Or I mean, how do you imagine language learning to be? There is a lot of reading, listening, watching, memorizing and writing involved. And grammar study. All of those are done alone.

Studying, programming, meditating, reading, writing, thinking, lucid dreaming...