How to integrate custom HTML template in a premium wordpress theme?

Your best bet is to figure out how to make a Child Theme for your premium theme. And learn how Templates work (and the template hierarchy) .

Start at the Codex . Lots of information about everything WP. And there are lots of googles/bings/ducks about creating themes and child themes.

With a Child Theme, you use the main theme as the starting point, then adjust things with CSS, hooks, filters, and templates to get things the way you want.

This is not an easy or quick ‘journey’, but lots of fun if you like tweaking and programming things.

Good luck!