1. Custom ‘Classic’ WordPress Theme
Most sites are powered by their own custom WordPress theme, always a modified (even more simplified) version of automattic’s original starter theme:
There’s always a functions.php for the site setup, then a header.php and footer.php that display on every page. The content of most pages is handled in a singular.php (standard pages and posts), and there is often also a front-page.php template, or other clearly named templates for other content variations.
This setup follows the basic WordPress recipe for a creating a Classic Theme. Some familiarity with this world would be helpful, but general experience with regular PHP programming should also make this architecture seem pretty straightforward.
https://developer.wordpress.org/themes/classic-themes/your-first-theme/#classic-theme
2. Advanced Custom Fields Pro
The main technology that makes all these sites more than just blogs is the use of the Advanced Custom Fields Pro plugin. Prior experience here, and definitely with the Repeater Field would unlock the ability to modify existing data or add new data in familiar ways to the designer and clients.
https://www.advancedcustomfields.com/pro/
3. CSS/HTML
Fluency in CSS and HTML is necessary – familiarity with responsive layouts and to be able to dynamically adapt and restructure the PHP/HTML content along with CSS changes. Sometimes styles can be superficially modified, but to be able to create new harmonious layouts and styles from scratch or from existing ones is a key skill.
4. Javascript
The level of Javascript needed is usually limited to simple DOM manipulations, tying things together, or solving one more tricky mobile layout request. No modern frameworks are in use. Most existing code is in jQuery, but vanilla JS is completely equivalent now. Needed for mobile menus and common requests for extending the capabilities of PHP and CSS one more level.