An Azure service that provides an event-driven serverless compute platform.
You could use Azure Logic Apps or Azure Functions.
For most scenarios where existing Biztalk systems need to be replaced, my Biztalk MVP colleagues are recommending Azure Logic Apps.
I'm a fan of Azure Functions but they don't offer the built-in options that you get with Azure Logic Apps. Conversion will still be a large task, but you'll get there faster with Logic Apps.
One note: I see people trying to do with with Power Automate. Don't do that. It's targeted more as an end-user tool than a server tool. Logic Apps is a server tool.