Free resource
Mandarin Blueprint (YouTube)
My notes
A memory-technique-focused channel — useful if you're trying to build a mental framework for retaining characters rather than just drilling them. I didn't use it as a primary resource but it's worth knowing about, especially early on when the character system is still unfamiliar.
Mandarin Blueprint's approach is built around memory techniques for character retention, which I found most useful in the early stages when the visual differentiation between similar characters was the main obstacle. I came back to it occasionally when a character I kept confusing with another was causing errors, and the mnemonic framework usually gave me something that stuck. Not a primary study method at upper HSK levels, but a useful diagnostic tool when specific characters are causing recurring problems that flashcards alone aren't solving.