From the Spaces layout, click on the Settings cog at the bottom left and click on 'Webdock layout'.
Choose the 'List Layout' from the pop-over. A pop-up message will be shown:
⚠️ List Layout is different from the Spaces layout because it displays Groups from different spaces in one big list. This is useful for power users who need lots of Spaces, e.g., for clients, so they can switch between them quickly and use split-view across apps and groups in different Spaces.
NOTE! If you switch from the Spaces view to the List view and then mix up your apps across groups in different spaces, returning to the Spaces view will be different.
In the List and Explorer layouts, you'll see a new Webdock picker icon in the top-left corner of the address bar. You can use this to switch between List and Explorer views.
To switch back to the Spaces layout, click on the Settings cog at the bottom left and select 'Spaces' again from the pop-out. (Note that the Webdock picker icon at the top left is no longer there.)