Three folders and a database

• wp-admin/ and wp-includes/, core. Never edit these; an update overwrites everything
• wp-content/, YOUR site: themes, plugins, uploads
• wp-config.php, database credentials and keys
• The database holds posts, pages, settings and users

Posts versus pages

Posts are dated and belong to a feed, news, blog. Pages are standalone, About, Contact. Custom post types cover the rest: Properties, Courses, Menu Items. Choosing correctly at the start saves a rebuild later.

.com is not .org

wordpress.com is a hosted service with limits. wordpress.org is the software you install on your own hosting, which is what client work almost always means.

Backup before you touch anything

Files AND database, both. A plugin update can break a live site in seconds, and "I'll just try it" on a client's site without a backup is how a good week ends badly.

Never edit core, and prefer not to edit a theme directly

Both get overwritten by updates. Customisations belong in a child theme or a small plugin, which is the next lesson.