can you share a few documents that teach the kotlin language really well and some prepared information about it?

probably google's own sources

As mentioned above, Google searching is your best bet.

One way I learned to program was also looking through existing, complex code

Since Kotlin is your interest, go to github and do a search for it

https://github.com/search?q=lang%3A+kotlin

awesome

And there probably exist dozens of youtube tutorials and you may also check out the giveaways/freebies section in this forum, maybe some udemy course or something "leaked" as well.