At this point, the new Layout Template system is "beta" and is running parallel to the existing Cards & Layouts system.
You can duplicate designs into the the new system, but the old designs remain available.
The new Layout Templates system has it's own Default Card, select it and edit as you wish. Change the name of this card if you like -- but note that when you look at mapping, the bottom card has a special icon, a bit like an open box with little blocks dropping into it. This "catch all" icon indicates the card used if no mapping rules apply.
The new Layout Templates system doesn't have a truly "Default" card sitting silently behind the mapping rules the way the older Cards & Layout does.
The last mapping rule is and shows the "catch all" icon.
You can drag rules up/down to change the application order, and there's a "catch all" icon at the top of the mapping column that you can drag/drop onto a rule to force it to the bottom of the rules and set it as the catch all rule.
The new system has a couple of new approaches, we're all learning them.