I'm interested in running 100% after beating the game yesterday, but I can't seem to find any tutorials on YouTube. Maybe I'm just blind, but could someone confirm that there are none? And if there are none, how should I learn it?

As with most runs, it's best to learn hands on in my opinion. Make sure you have most basics down from other tutorials and apply what you learn there to any run you see. Practice is definitely the most important part no matter what. I'd definitely make sure you're familiar with how long to ground yourself to collect a berry and which ones you definitely don't want to lose by dying after collecting. I find it wise to break it into chunks, so full clear ILs might benefit you greatly. If there is a tutorial out there, I'm sure it would also be useful.

K. Also what would you recommend for getting better at mechanics? I've learned them but I can't get extended dashes consistently, wall boosts take 10-20 tries (for the hard ones, easy ones take 2-4) and whenever I need to use a type of dash (when im following an any% tutorial) I always end up not going for enough or not jumping high enough and things like that.

Ignoring the adage "practice makes perfect", it helps to identify why you aren't executing the techniques correctly. Let's take extended dashes, and I'll assume hyper/wavedashes for this example. If you're not getting your dash back, you're probably pressing jump too quickly. 99% of the time, the spot you choose to hyper/wavedash should be suitable to be extended. Similarly, for wall bounces, if you're not actually getting the bounce part, you're a little too far from the wall OR you pressed jump too late and you're getting a normal wall jump. I'd offer a little more precise advice (assuming I know exactly why, as I don't know everything yet), if you had any footage of your practice. Regardless, figuring out WHY you're not getting the tech right goes a long way to fixing it up.

I'd recommend for practice with general movement, you go to the Awake section of chapter 2, and practice there. Pretty much no way of dying there, and plenty of open space to mess around.

Also, I wouldn't exactly know which any% runs you watch, but doing a test run and watching someone with a similar time (and ideally similar skill level) would be useful to learn from.