Viewing the content of multiple projects at once can be very useful for when you need to quickly select multiple items at once from various projects to reorganize or delete.
To view the content of multiple projects at once, you must switch to "Content View" while viewing all projects.
First, select "Projects" on the left side-panel to view all projects.
Then click the grid icon in the upper-right to switch to "Content View".
You will now see all the content in your account at once, grouped by project.