Standard problems are standard

standard problems are standard because they are well-known (standard)

Maybe it's a prob that is easy?

probably bc they only of x topic or the sol is just x + minor things

I think this is slightly less simple then you think. There are standard problems, standard algorithms, standard reductions, and standard final solution ideas. All of which standard means "common" or "well-known". Standard problems you can find on sites like cses, standard algos on sites like cp-algo, but there are not quite so well organized lists for the latter two, even tho if u gave an example to an experienced participant they'll generally agree if it's standard.

You mostly learn the latter by being able to generalize and understand commonalities through problems you practice, and they are what is most important to the cf meta now (often can guess reduction, guess what final solution will look like because they keep repeating based on setup). For some reason people like this.

A problem is standard if it has already appeared in Atcoder ABCs especially E's and F's.