Present a modal with a custom view (ie. present a view controller using a xib)

These are the articles that helped me:

The nitty gritty step by step details are in this blog post.

Key steps

Add a xib file and a .swift file with the same name
Make sure the xib file has a UIView and not a UIView Controller. If you have a view controller in the xib, then you will get the error: A view can only be associated with at most one view controller at a time!
In the xib file, define the custom class for the file owner, not the view

Prior to Xcode 12

Connect all the outlets from the xib file directly to the swift file.
In the xib file, click on file owner, then outlets, then drag and drop the view outlet to the view. If you don't do this, you will get the error: loadViewFromNibNamed:bundle: loaded the nib but the view outlet was not set.

Xcode 12

The "view" outlet in the xib file > file owner no longer exists. However all the individual UI elements are now present in the file owner.
In the xib file, click on file owner, then outlets, then drag and drop all the outlets as required from the right hand nav to the correct UI element in the nib. e.g. connect the label to the label, the button to the button.
If the app crashes with an error similar to UIelement is not key value coding compliant, then you probably have connected that element in the file owner AND in the view. Delete the connection that is sitting in the view.

Blank view? Don't forget you need initialisers in the .swift file. If you forget this step, then you will see a blank view. Nothing will crash, no warnings, but you won't see your custom view without initialisers.