How to create simple template for joomla 4
Cassieopia template Joomla 4 is equivalent to Protostar in J3, and is just as simple to configure. You could duplicate that, and then study the code to see how it is created - it's probably more complicated than you realise (from a coding perspective).
J3 came out of simpler times and so it has Protostar, a simpler template. Maybe eventually someone may even adapt Protostar to be J4 compatible.
That said, I can't find much on "creating J4 templates" - every web search comes back to using a framework like T4, Helix, Gantry etc. (I suppose these days, that's how templates for CMS's are done).
By the way you can download blank template Joomla 4 here