How to create custom page template suitable for any new page

I;’m not too clear on what you are trying to achieve. But to answer your specific question, you can target particular pages or categories for example using is_page, or is_category using conditional tags. There are lots available :
https://developer.www.ads-software.com/themes/basics/conditional-tags/

There are also plugins to target your CSS based on conditional criteria so you’re not loading it all the time :
https://www.ads-software.com/plugins/wp-custom-body-class/

To add on what corrinarusso said, there is also the template hierarchy doc page that may help a little: https://developer.www.ads-software.com/themes/basics/template-hierarchy/

It may lend some idea on what file will be used when post, page, or custom post type is queried.

Hope that helps out in some way

This is a handy plugin as well:
https://www.ads-software.com/plugins/which-template-am-i/

thanks so much for your helpful replies ??

Of course!

Happy to help.