"These are they" is usually correct, though "these are them" would be accepted by all but the most pedantic. This is, as you note, in accordance with the normal rules whereby a plural noun or pronoun takes a plural verb-form.

That's true for the common case, when "they" refers to a group of things or people. However, "they" is a little more complex than other pronouns, in that can be either singular or plural. When using singular "they", as a non-gendered pronoun for a single person, most native speakers would prefer "this is them". Again, "this is they" might be more strictly grammatical, just as "this is he" is more strictly correct than "this is him", but in everyday language that distinction doesn't matter.

"These are they" is usually correct, though "these are them" would be accepted by all but the most pedantic. This is, as you note, in accordance with the normal rules whereby a plural noun or pronoun takes a plural verb-form.

That's true for the common case, when "they" refers to a group of things or people. However, "they" is a little more complex than most other pronouns, in that (like "you") it can be either singular or plural. When using singular "they", as a non-gendered pronoun for a single person, most native speakers would prefer "this is them". Again, "this is they" might be more strictly grammatical, just as "this is he" is more strictly correct than "this is him", but in everyday language that distinction doesn't matter.