Do "symbols" = terminals in CFGs?
In a context-free grammar there are two sets of symbols (two alphabets, if you will). One is the set of symbols that can be replaced by others: these are the non-terminal symbols. The other is the set of terminal symbols, which (you may have guessed) cannot be replaced.

At some point, a derivation is expected to produce a string of symbols that is immutable, and because it is immutable we tend to think of these strings in the same way we think of numbers (as opposed to arithmetic expressions), as elements of a language that is being defined. This is one game that we play.

You can still think of those two sets of symbols as partitions of a bigger set, the "set of all symbols". This would allow you to play other interesting games.