You have not fully understood what the dictionary is saying.
“I saw you somewhere in France.”
Somewhere is a singular noun and an adverb:
“I saw you somewhere (adv.) in France.” = at an unknown/unspecified place.
“The ship has come from somewhere (n.) and is going to somewhere(n.) = an unknown/unspecified place.
The plural of “somewhere(n.)” is “some places”. - two words
“The ship has come from some places in Africa.”
“The battles were fought at some places in France.”
Something is a singular noun:
“Something(n.) is making a noise.” / “I need something(n.) to write with.” = an unknown/unspecified thing /object.
The plural of “something (n.) is “some things” – two words
“I will put some things into the attic.”
Somebody and someone are nouns:
“Somebody(n.) / Someone(n.) is making a noise.” / “I need somebody(n.)/ someone(n.) to help me.” = an unknown/unspecified person.
The plural of “somebody (n.) / someone(n.) is “some people” – two words
“Some people are making a noise.” / “I need some people to help me.”