Hilary Hahn herself has been doing '100 days of practice' again recently - she's on day 87 now. Another example I've been enjoying lately is the German pianist Annique Goettler practicing sections of the Moszkowski piano concerto - sometimes she does split screen and also plays a reduction of the orchestral part (on Instagram, also her YouTube "Heart of the Keys" account). She's a pro but has a sense of humor about the ridiculous amount of messy work and frustration it takes to get things polished. See her recent "1 Min, 10 Min, 1 Hour Challenge" for the last movement of the moonlight sonata.

This is the kind of thing that in the old days you would only see if you hung around a musician's house. I used to have an older pianist friend who would say "look what I'm working on!" and then crash through some crazy virtuoso piece that months later he would play perfectly. Most kids don't have direct access to musicians at that level, so I really appreciate this trend of practice videos. Plus, seeing behind the curtain makes the music even more interesting.

Some of the people I enjoy listening to (almost all violin):

https://www.instagram.com/nancyzhou_violin/ https://www.instagram.com/joshuabrownviolinist/ https://www.instagram.com/kersonleong/ https://www.instagram.com/violincase/ https://www.instagram.com/augustinhadelich/ https://www.instagram.com/weatherclef/

I also enjoy just browsing the discover tab on instagram. If you mostly click on these instrumentalist accounts, it's pretty good about finding more.

The instagram page of Lauren Pierce (bass) is a lot of fun for me since it's encouraging to watch a player of her caliber struggling with really difficult pieces, or working on pieces she doesn't enjoy but still motoring through them anyway.

Not specific accounts but Reddit Public Access Network and Twitch are interesting services to discover this kind of content. Although the signal to noise ratio is low.