Ce sont

Stephen, 

see the previous answers linked below for fuller discussion. ( The links seem to be showing the same description as I post this, but are to 2 different previous QandAs )

Briefly,  ‘ ce ‘ is an invariable demonstrative pronoun and the subject in the expressions ‘ c’est ‘ and ‘ ce sont ‘. 

This is not the masculine adjective ‘ ce ‘ which, along with ‘ cet/cette ‘, takes the common plural form ‘ ces ‘. (See Larousse links for detail ).

There is no expression ‘ ces sont ’, as it makes no sense and is grammatically incorrect in French as that would an adjective-verb combination. The word ‘ ces ‘ is only ever a plural adjective, and precedes a plural noun.

 https://progress.lawlessfrench.com/questions/view/why-ce-sont-devenues 

 https://progress.lawlessfrench.com/questions/view/why-ce-sont-devenues

 https://www.larousse.fr/dictionnaires/francais/ce/13946

 https://www.larousse.fr/dictionnaires/francais/ce/13947

As so often, that’s just the way it is, ce sont, is the correct phrase. To make things more complicated a) c’est can be used in colloquial spoken French for plural nouns (c’est mes amis) and b) for making a general comment about singular or plural nouns, with an invariable masculine singular adjective.

 Lawless goes into it fully: 

https://www.lawlessfrench.com/expressions/cest/ 

https://www.lawlessfrench.com/grammar/cest-vs-il-est/