Udemy is great, I agree with the most of commenters. Talking about only Math, there are many special printable materials (I personally like worksheets from https://myschoolsmath.com/ , they’re printable and free, seize all the studying grades). Happily today it is not a problem to upgrade any skills online.
I'd agree that online education is more preferable but it has the same problems as an ordinary one. Ineffective and unnecessary homework and writing tasks. To deal with it I recommend this service https://essayspirit.com. With its help, you'll lose lots of boring tasks.
Hello! it always depends on final purposes. I mean, that even Youtube can be considered as a valid learning platform. To get into software engineering or you just wanna know all features of MS apps I'd recommend you Infoplus service. It has a good amount of various guides and articles. For instance, https://www.infopulse.com/blog/using-microsoft-graph-api-inside-microsoft-flow-office-365/ where you can learn to use MS graph api.