Over the last ten years or so, I have worked with many different computer-based training (CBT) programs from various vendors, some paid, and some free. Of all the ones I’ve used, Microsoft Learn – which is free, as in “free beer”! – has been the best overall.
Not only is the variety of subjects available for study more diverse than the other sites I’ve used (though the Learn classes are all for only Microsoft-based technologies, as far as I know), but the quality of the lessons and the degree for which they can prepare one for at least some certifications is comparable with a paid class or set of Microsoft Press books.
I was particularly happy with the Power BI and Azure training that I took, and I plan to move on to some AI and Machine Learning courses this next year.