A Microsoft open-source framework for building native device applications spanning mobile, tablet, and desktop.
The announcement from Microsoft shares some recommended replacements for each App Center capability: https://learn.microsoft.com/en-us/appcenter/retirement
For building they recommend Azure Pipelines, although I'd personally recommend GitHub Actions paired with Fastlane as a good pipeline for automating signing, and deployment to App Store / TestFlight / Google Play.
For replacing App Center's beta distribution capabilities, if you don't want to rely on platform specific options like deploying to TestFlight or Google Play Console directly, there are tools like Buildstash which integrate with Azure DevOps Pipelines or GitHub Actions for distribution.