I want template xyz to work with multiple pages

The templates were originally invented to be one-page summaries of your CV. There are some templates with multipage support out-of-the box:

• classic
• modern
• rows

A special case is the sidebar-left template, which implements specifically support for multiple pages. If you want to see how you should layout your template to support multiple pages then you should take a look at it's code.

It uses the paracol environment to realize a multipage-multicolumn layout, where each column has it's own page-flow, so contents in each column break independently from each other.

See:

https://www.ctan.org/pkg/paracol https://tex.stackexchange.com/questions/497049/three-column-layout