Where do I start with building a plug-in integration system?

Hi hackers 👋

I’m testing out an idea for a simple plug-in that integrates with calendar.

Looking to build a mock-up and then of course the real thing so I’m in the early idea stages.

I’m familiar with their benefits like those on the Shopify app store for example, that allow easy integration with stores and give them different capabilities, but I’m not sure how they’re built.

My question is where do I even start with building it? How is a plug-in usually built, like on what language(s)?
Or maybe I’m lucky and there’s a No-Code tool I could use to build it quicker to validate?

Thanks in advance!