Docker vs VM question - little more focued question

I have read numerous docker vs vm postings on numerous sites; however, there is one question that I can’t seem to find the answer to. Yet, it may be obvious once I hear (read) it. Assume you have A) a docker container running Ubuntu and B) a vm running Ubuntu. The following questions come to mind:

1 - Aside from performance, how can I expect these two Ubuntu instances to behave differently?

2 - Is one more stable than the other? Is one more likely to cause problems than the other?

3 - In a typical (for me anyway) scenario where you have a web app consisting of a javascript client, node js server, Postgres backend database. Is there any reason to prefer one solution (A or B) over the other?

4 - Finally, considering the “docker is not a vm statement that I hear endlessly”, how is docker able to run not just an application in a container but rather an OS (ubuntu) that can itself run multiple processes?