During bidding, a sub sends questions to the general contractor, who sends them to the architect. The instructions to bidders give the deadline for questions.
A good RFI is specific. For example: the fixture schedule lists type P-3, but sheet P-201 shows no P-3. Please confirm the quantity.
Name the sheet, the detail and the spec paragraph. Ask one question per RFI. Suggest an answer if you have one, because that speeds the reply.
An answer only changes the contract when an addendum carries it. If no answer comes, state your assumption as a qualification.