What does the iTunes backup of the iPhone include and what does it not include on Windows 10?
straight from Apple themselves, as the first result on your Google search;

A computer backup of your device, which is not the same as a sync, includes almost all of your device's data and settings. A backup from a computer doesn't include:
Content from the iTunes and App Stores, or PDFs downloaded directly to Apple Books; Content synced from Finder or iTunes, like imported MP3s or CDs, videos, books, and photos; Data already stored in iCloud, like iCloud Photos, iMessages, and text (SMS) and multimedia (MMS) messages; Face ID or Touch ID settings; Apple Pay information and settings; Apple Mail data; Activity, Health, and Keychain data (to back up this content, you'll need to use Encrypted Backup in iTunes.)

your backup data should be located, by-the-by, at %appdata%\Apple Computer\MobileSync\Backup\.
the TL;DR is that a full iPhone backup will only actually back up data and settings important to the iPhone, such as keyboard layouts, dark/light mode settings, language preference, and more.
to back up your photos, music, contacts, and whatever &quot;other&quot; is classified as, you're looking for backing up with iCloud, which (unless manually disabled in Settings) is automatically done.