Subs receive bid invitations by email from general contractors, often through a plan room. A public owner publishes a formal invitation to bid.
The invitation gives the project name, the location, the bid date and a link to the documents. It may name the trades wanted.
Check four things first: the bid date, the scope for your trade, the project size and who else is bidding. Then decide to bid or pass.
Reply either way. A GC that knows you will bid keeps sending you addenda. A GC that hears nothing may stop inviting you.