fastening vs joining

fastening and joining both are nouns.

Word	Noun	Adjective	Verb	Adverb
fastening	Yes	No	No	No
joining	Yes	No	No	No

As nouns, joining is a hypernym of fastening; that is, joining is a word with a broader meaning than fastening:

fastening: the act of fastening things together
joining: the act of bringing two things into contact (especially for communication)

Other hypernyms of fastening include connection, connexion.

fastening (noun)	joining (noun)
the act of fastening things together	the act of bringing two things into contact (especially for communication)
restraint that attaches to something or holds something in place