Much depends on how each theme is designed. Ad space slots are available in both premium themes and some free themes, plus of course, if you design your own theme, or have your custom own theme designed, then you can decide from the outset if you want ad space and where and how your ads appear.
For example, the free TechNews theme at: https://marketplace.getpublii.com/themes/technews/ includes options to insert your ad code snippets in the right side column, plus on the home page main content area, tag pages, and author pages. Check out: https://getpublii.com/themes/demo/technews/
Or the premium N-Tech theme at: https://marketplace.getpublii.com/themes/n-tech/ includes a superb range of predesigned ad slot options – scroll down the page to to see them.
