Quick answer: alle is everyone and alles is everything.
The whole story is more complicated and judging by websites on this subject not all native German understand the fine details. Here's a You tube vid to get you started (although don't try to copy his strong Ami-accent) https://www.youtube.com/watch?v=nfxbcG9BeSY
Jein :-) Your sentence is wrong in English because "everyone" means "each person" and must be singular. But, I think, you're reasoning along the right lines in that "mögen" in this sentence implies that "Alle" is plural. A way to force the plural in Englis is to say "we/they all like him"
In the lesson notes it says mögen is used for "people, animals, and things"; does it mean to say that it is used for nouns more generally or can it not be used for abstract nouns? eg Can one say "Ich mag... quiet/happiness/tranquility" etc?
(Please excuse the English substitutes here! I am not far enough through the course to have adequate vocabulary to present these words in German yet!)