https://www.packtpub.com/packt/offers/free-learning
Packt publishes a ton of technology books, they also give one away in PDF form every day. This week appears to be all Unity themed books, so if you’re interested in some free high-quality Unity guides it’s worth making an account and grabbing them.
I’m sure they don’t compare to the awesome Udemy courses but always good to supplement and reinforce your knowledge!