These days I'm creating a lot of mobile applications for different clients with/without an API. Mobile applications are done using Ionic or Flutter depending on clients need. Both are pretty good choices. For those without API, I build a new API from scratch typically using .NET Core and then you can run that on any machine because .NET Core is platform agnostic, it can run in a Linux, Mac or Windows. Plus it is widely supported by AWS and Azure so it can be put in a Docker and scaled.