The theme is how your WordPress displays your content. Most simply… it manages the structure, arrangement, size, color and interactions with your website.
There are MANY themes in the marketplace. Additionally there are pre-packaged themes from vendors that can be installed as well.
Some like to use “theme-builder” plugins like elementor, divi and beaver builder. These are comprehensive plugins that include tools for very specific details modification of your layout and theme functionality.
Keep in mind that your theme should be updated regularly to make sure it’s secure and keeping up with new industry changes in php code and security. This is why I get anxious about using a pre-packaged theme from a small vendor. Yes it might be exactly how you want it to look, and it might be cheap; but 3-5 years from now are they going to make sure updates are provided?
For my basic sites and lean SEO targeting I like using the default WordPress themes. You know they will be updated and they have grown more and more customizable.
For my pretty and visually impactful sites I will use Divi but I try not to get carried away with the options.
Do not use a bunch of theme-builder combinations; this will just slow down your site and add complications if there’s a major update.