create a function in PageController.php that returns a theme that points to a template and whatever variables you want
define the template in the hook_theme() in the .module file
define routing configuration in routing.yml

To define a custom block:

create a block php file in src/Plugin/Block that returns a theme that points to a template and whatever variables you want (similar to the Controller above) OR return a simple #markup string if you don't need anything complex.

iDC Serializer

This module, and the iDC Serializer that is part of this module, incorporates, modifies and otherwise combines parts of the Facets and Pager Serializer projects. The date these modifications were first incorporated into this module is March 9, 2021.