If someone wants to create a project by assembling bits and pieces from different open source products they can, but many just go for Spring (Boot) and call it a day.

All of my projects are based on Spring and I don't really have to look outside of that ecosystem. It almost acts as an aggregator of different open source solutions and often works by abstracting the functionality so that differences are not that big. I recently switched messaging providers and didn't have to change much of my code.

Yep, with Spring Boot development is so easy, and even decade old projects are mostly easy to upgrade. I don't have experience from C# or .NET development, but at least compared to Python and especially JS ecosystems it's so much better.