join vs assemble

join is a noun but assemble is not a noun.

Word	Noun	Adjective	Verb	Adverb
join	Yes	No	Yes	No
assemble	No	No	Yes	No

As verbs, assemble is a hyponym of join; that is, assemble is a word with a more specific, narrower meaning than join:

join (noun)	assemble (noun)
a set containing all and only the members of two or more given sets
the shape or manner in which things come together and a connection is made

join (verb)	assemble (verb)
make contact or come together	create by putting components or members together
cause to become joined or linked	collect in one place
become part of; become a member of a group or organization	get people together
come into the company of
be or become joined or united or linked