When I double click on my object, the properties don't show up on the side pane. How can I bring them back?
With the disappearance of the Properties window since version 2018, all categories from the Side pane have the option to float away from the property pane to drag it around one or several screens if you have multiple displays. Likewise, they have the option to be attached back into the pane.
Because the floating window is a separate entity, it is possible for it to become visibly lost from your screen especially if using multiple monitors.
Whenever you cannot see any of the properties from the side Pane after double clicking on a part or command, it is likely that the window is lost. You can quickly bring it back by right-clicking on the property pane and selecting "Attach to side pane." See the linked article under the section "Float and dock a side pane window."
This option will only be available after activating the properties. To enable, either double click on your part or click on the properties' icon once and then right-click.