As far as I know, "ellos" is not considered a demonstrative pronoun. You can see a list of demonstratives in Wikipedia. For the third person plural we have: estos, esos, and aquellos.

You can use any of them for your second sentence depending on the spatial or temporal location.

¿Donde viste a esos?

Also you can use "los" as well in that sentence

¿Donde los viste?

So in Spanish that inflection that you mentioned stands for the third person both in singular and in plural.

As far as I know, "ellos" is not considered a demonstrative pronoun. You can see a list of demonstratives in Wikipedia. For the third person plural we have: estos, esos, and aquellos.

You can use any of them for your second sentence depending on the spatial or temporal location.

¿Donde viste a esos?

Also you can use "los" as well in that sentence

¿Donde los viste?

We make the distinction between they and them using "a" before "ellos" (like in my example). So there is a distinction that does not imply changing the word but using additional ones. More info (in Spanish) about the use of that "a" (a preposition) in direct objects here

On the other hand, demonstrative pronouns are not used mandatorily as direct objects, you can use "regular" pronouns.