bus vs public transport

bus and public transport both are nouns.

Word	Noun	Adjective	Verb	Adverb
bus	Yes	No	Yes	No
public transport	Yes	No	No	No

As nouns, public transport is a hypernym of bus; that is, public transport is a word with a broader meaning than bus:

bus (noun)	public transport (noun)
a vehicle carrying many passengers; used for public transport	conveyance for passengers or mail or freight
a car that is old and unreliable
an electrical conductor that makes a common connection between several circuits
the topology of a network whose components are connected by a busbar

bus (verb)	public transport (verb)
remove used dishes from the table in restaurants
ride in a bus
send or move around by bus