What is the difference between synecdoche and metonymy?

synecdoche sounds like Schenectady = part represent a whole
metonymy = substitution

Whaaat?

Glad I did the spelling bee :D
I think the difference is that synecdoche takes a word and attributes it to a kind of whole while metonymy was maybe like just changing the place of a word for a synonym kinda 🤔

I have no idea what either of them words are

They are the names of my dogs.

One letter y