Plugin support in modern OpenSource software?

Hi all, there is one thing I have been asking myself ... and I realized it makes a ton more sense to ask others at this point because I don't even know what search terms could help me find answers.

Imagine you wanted to create a WordPress competitor with modern technology that is scalable (I am thinking microservices, with web component support in the UI etc.) ...

are there any technologies or concepts that you would look into (or that you already know you would use) to implement a convenient Module / Plugin mechanism?

When I play the thought I get stuck, because I only know how features can be added through code changes.

With PHP I can see how a plugin can affect the full stack. With microservices this inevitably seems like a complicated operations topic that also turns the software admin into a system admin.

Is there a way to implement (full stack) Module / Plugin support that allows a 3rd party developer community to plug into extension points for standard software like phpBB or WordPress?

Where should I start a discovery phase?

Question from: https://www.indiehackers.com/forum/tech-question-plugin-support-in-modern-opensource-software-6a9632d412

I thought you guys could have some great answers.