Plan rooms were once physical rooms with paper sets. Today builders exchanges, reprographics firms and online bid platforms host them. General contractors also run private ones for their invited bidders.
Example: a subcontractor gets an invitation with a link. The link opens a plan room page with drawings, the Project Manual and addenda as separate files.
Download the full set, not only the sheets filed under your trade. Check the plan room again before bid day for new addenda. Some plan rooms list the bidding general contractors, which tells you where to send your price.