Windows' default app settings decide which apps launch automatically when you open a file or perform certain tasks. Out of the box, Windows will use its built-in apps, so you'll get Microsoft Edge for web browsing, the Mail app for email and Photos for image viewing. These options can be changed though, letting you ensure tasks are always completed using your favourite apps.
Once you've installed your desired apps, head to the Settings app to start changing your default applications. Click the "Apps" category from the homepage and then navigate to the "Default apps" page within the section that opens.
At the top of the page, you can select your default app for each of six common tasks – email, maps, music, photos, videos and web browsing. The currently configured app is shown by default. To make a change, click the app's name.
A menu will appear showing all the installed apps associated with the task category – media players will be offered for the "music player" category, while email clients will be shown for "Email." Click the app you want to use as your default to configure it.
The impact of the change will vary depending on the app category. In general, default apps will be used when an app needs to be opened by the system or another program. For example, the default web browser will open links you click in your email app, while the default email client is the one that will draft new messages when you click an email address on a webpage.
You can get more fine-grained control over your default apps using the three links at the bottom of the page. The first two, "Choose default applications by file type" and "Choose default applications by protocol," give you granular control over the circumstances in which an app should be considered the default.
If you want Microsoft Photos to be your default photos app, but need JPEG files to always open in Adobe Photoshop, you can configure this with a file type association. Find the ".jpeg" file extension in the "Choose default applications by file type" list and click the displayed app name to change the default.
Finally, the bottom-most link, "Set defaults by app," displays a list of all your installed apps. Clicking an app and then pressing the "Manage" button will reveal a list of all the file types and protocols that it can act as the default for. This gives you a simple way to make an app the default for all the file types it supports.
It's worth noting that you'll generally only need the main "Default apps" page in settings, unless you're an advanced user looking for more precise control. Many program installers let you register new apps as defaults for their category or file types during setup, so look carefully for these options when you add new software. If you want to change these defaults later, or reverse an unintended change made by a program installer, just head back to the "Default apps" page in Settings.