The word, "phrase “getting on", has various different meanings in British English.

“Getting on” has more than one word, so it is a phrase.
Commas don’t make sense here as you would only use them if the reader already had some idea of what phrase you are referring to.
“Various meanings” is grammatical, but adding “different” is better so that you emphasize why it creates confusion.

I only knew the meaning of “riding”, but it can also has the usages for be used to mean “dealing with”, “getting along with”, “going on”, “getting old”, and “getting late”, as far as I fihave found out.

English phrases are difficult for me because it's often hard to guess what it saysthey mean even if I know these individual words.

“These words” is somewhat vague, as it would be referring to a specific phrase or set of phrases, and you could also be referring to the meaning of a phrase as a whole.

When I find phrases I don't know, I add them in an Anki deck little by littlone by one.

“One by one” sounds better to me because “little by little” could mean that you just add them in at a very slow pace, in my opinion. The meaning is a little different though.