Owners send invitations to general contractors or post them publicly. General contractors send their own invitations to subcontractors and suppliers, often through a plan room or an online bid platform.
Example: a general contractor invites a specialty supplier to bid one trade. The invitation gives the bid due date and a link to the files.
Check the date, the trades requested and the documents attached. The general contractor's due time for sub bids is often earlier than the owner's bid time. Reply to the invitation, even if you decline. Estimators remember who answers.