"Live Preview" plugin

Hi everyone. I hope you guys are well.
I am starting a website development side business, and one thing I would love to offer my future clients is the option to select a template directly on my site.

I have no idea how other sites do it, but I would like to have a LIVE PREVIEW of the templates I offer.

Can you point me in the right direction for a plugin (free or premium) or article on how to set this up?

Thank you.

Andre Costa