Usage of 'be to' like "is to"

Question

They have something a bit different, which is to think up an idea for a five-minute movie and then film it, using special animation software.

— Cambridge IELTS 16 Listening Test 01

I don't know what kind of grammar is used in bold text.

Answer 1

There is no meaning to "is to".
You should look at the meaning like this :: "which is" & "to think up" ;; not like "is to".

They have something a bit different, ...
... which is
...... to think up an idea
......... for a five-minute movie
...... and then film it,
......... using special animation software.