Blueprint Exchange
The blueprint exchange is an in-game repository of blueprints (designs) published by yourself and other players through Steam.
The blueprint exchange is integrated with the Steam Workshop, which allows you to use a published blueprint right away without ever needing to look at the designer yourself.
Note: It seems that the Steam Workshop page isn't accessible unless you're logged into a Steam account which owns Starship.
Building Blueprint Exchange
The building blueprint exchange handles building blueprints, and is accessible by hovering over the Building (F10) button on the sidebar.
Saving building blueprints from the blueprint exchange to media is not necessary. The Construction (F11) window can be configured to show all published and subscribed building blueprints that can be used for construction.
Spacecraft Blueprint Exchange
The spacecraft exchange is a place where users can submit and download finished spacecraft blueprints and partial assemblies. It is accessible from the Spacecraft (F9) window's Exchange tab. You can also hover over the Spacecraft (F9) button on the sidebar.
Publishing
A finalized blueprint can be published to its corresponding blueprint exchange with an "access level" option. With this, you can allow/disallow other users who obtain it from the Steam Workshop from editing the blueprint.
When burning a blueprint, it is added to whatever media you select. From this, a spacecraft blueprint can be manufactured at a spacecraft factory, or building blueprint can be selected from the Construction (F11) window's blueprint drop-down. Do note that buildings do not need to be burned to gear to be constructed, only available in the exchange.
Blueprints burned to a media straight from the designer are not published here. A blueprint does not have to be published to be useable, and once downloaded from the exchange the blueprint will always be useable even if it is unpublished.
Steam Workshop
On the 7th February 2023, Hazeron Starship's exchanges were integrated with the Steam Workshop.[1][2]
This allows players to publicly upload, view, download, comment on and rate both spacecraft and building blueprints, mostly through the respective in-game blueprint exchange UIs. They can also be accessed as normal on the Steam UI, including editing uploaded blueprints. Ratings on workshop items are shown in-game on the exchange listing, as a 1-5 star value. Tags are automatically applied to the items depending on what the blueprint contains - so a spacecraft with Gravity Drives and a Vulcium Hull would have those two things tagged separately. Access level (full/restricted) is also tagged.
Do note that the local exchange still exists, and the Steam Workshop part of it is purely optional through the new buttons dedicated to that.
How to download
To use workshop blueprints, simply subscribe to any listed building blueprint using the green arrow button and you can build it. However, Spacecraft need to both be subscribed to and burned to a Storage Media Device before construction.