So I found an old ass website in which one can see at a glance a bunch of mathematical rules so a layman like me can start understanding all those letters and formulas for once.

The site is this:
http://hyperphysics.phy-astr.gsu.edu/hbase/hmat.html#hmath

My question is, do you guys know another website like this but more complete and modern? Modern is not even necessary.