Media Player

Media Player is the default audio and video app in Windows 11 and the modern replacement for Windows Media Player. It lets you play downloaded and locally stored audio and video files and can help you manage your personal music and video collections. This can include content you've purchased online, ripped from CD, DVD, or Blu-ray, or downloaded from the Internet and now store on your PC or on other PCs on your home network.

Tip: Windows Media Player Legacy is still available in Windows 11 if you're nostalgic for the early 2000s.

Tip: Media Player can only manage or play content that is unprotected by Digital Rights Management (DRM) technologies. So it cannot access the movies and other videos you may have purchased from online services like Apple or Google or music from subscription music services like Apple Music, Spotify, or YouTube Music.
Defaults
Media Player is the default in-box app for audio and video files (in .aac, .mp3, .flac, .m4a, .mp4, and many other formats) in Windows 11. In this role, the app is adequate and works as expected.

Media Player can also be used to manage local music and video collections, an infrequent need these days thanks to the popularity of music streaming services. If you maintain local music and video collections, it's unlikely that Media Player will meet your needs, but it monitors your Music and Video folders by default and populates the related views in the app with that content.
Customize
Media Player is a simple app with a modern design. There's little you can do to customize the app layout, as the navigation bar on the left is non-resizable and will automatically collapse as needed when the window resizes. But you can't toggle the navigation bar between expanded and collapsed views, nor can you toggle or resize the playback controls area at the bottom.

To customize Media Player, open the app's settings by clicking the "Settings" ("gear") button at the bottom of the navigation bar.

The following settings are of note:

Music library locations. By default, Media Player will monitor your Music folder (C:\Users\user-name\Music) for content. You can add a new folder to monitor by clicking "Add folder." Or you can expand this item to view or remove the folders it monitors.
Video library locations. This works identically to the Music library, except that the app looks for videos only in your Videos folder (C:\Users\user-name\Videos) by default. You can remove and add locations here as needed, of course.
App theme. Like many other Windows 11 apps, Media Player uses the system-wide color mode (Light theme or Dark theme) that you configure in the Settings app in Personalization > Colors by default. But you can change that here.
Accent color. Media Player is also unique among the in-box Windows 11 apps in that it uses its own fun accent color, called "Zest," by default. But you can configure the app to use the system accent color like other apps if you prefer...