There are 2 ways to have a project hidden from the public:
Unlisted Projects: Disable visibility from the public, but allow participation
Preview Link: Disable visibility from the public, allow the project to remain in draft, used for testing purposes
Unlisted Projects
By default a project is set as "Listed". This means it will appear on the platform, as a normal project where only the access rights and visibility rights from the project/phase settings will impact the way the project appears to the end-user.
When you "Unlist" a project, it is now no longer visible to the public, and the only way to access it, is by receiving the URL to the project. There is no other way to navigate towards the project. Therefore it will also no longer be part of the homepage and widgets that include project visibility.
This feature is used to allow Admins and Project Managers to have a project that is only accessible to a target group of users. You can find the toggle to enable or disable listing in the 'General' tab of the project (see image)
Note: You can both list and unlist a project at any time.
Preview Links
Preview links are used to share projects with other administrators or project managers to share a project, even when it is in draft. This way people can test the project before it's going live. You can find more information on how to set this up in this support article.