How to run automated Tests in Azure Test Plans

I want to be able to run automated tests from within Azure Test Plans and define everything in a yml file.

What I've done: I have a repository in azure DevOps containing several tests written in .NET core. Those are associated to a testcase in azure test plans. I also have created a azure pipeline by adding a .yml file to my repo. This pipeline consists of two stages, one for the build and one which actually runs all tests on a nightly schedule.

However, I want the manual testers to be able to trigger the automated tests from within Azure Test Plans too. The official docs say one must create a release pipeline to do so. So my questions are:

1. Why do I need a release pipeline? I already have my build pipeline, I won't do any release, I just want to be able to run my tests from within test plans by button click.

2. If I really have to create release pipeline, how can i define this in a yml file? The official docs only mention the "Classic", manual steps within azure DevOps, but I don't want to add pipelines manually, I want to have everything defined in yml files.

3. Is there a possibility to run my tests containerized? I do it already in my build pipeline, but since I couldn't find a way to define release pipelines in yml, I don't know how to run them in docker containers.