What are ways to bridge different services/stacks together?

I'm not sure yet of the interface eg. a native app or web app/PWA regarding being able to "add on modules".

For example one tool I made for myself is a CRON-remind me SMS(Twilio) it's using PHP. I want to modify this(why use SMS I know use push notifications) so I can embed it in an app (type mentioned above).

So it can be called within the app but still work on SMS. Other things like TTS(I use AWS Polly, generates audio files from news snippets). A specific credit card calculator… Etc…

If you wanted a unified app that uses all these things but written on different tech. "zappier" "REST/JSON"

I'm mostly looking for design approach regarding the modules part/services