They're pretty much whatever the company defines them as.
To the best of my knowledge, 'Release Manager' and 'Delivery Manager' don't come from any published frameworks, the way 'Scrum Master' comes from the Scrum Guide.
So, the answer to all three of your questions is 'it depends how your company defines them'. There may be common interpretations, but there'...
So does PO need to attend every Sprint Demo?
The Product Owner should be getting visibility of the work being done by the team throughout the sprint.
The intention of the sprint review ceremony in Scrum is not to inform the Product Owner of the work that the team has done, instead it is to inform stakeholders who are not involved with the team day-to-day ...
In a nutshell, Scrum is explained like so (emphasis mine):
A Product Owner orders the work for a complex problem into a Product Backlog.
The Scrum Team turns a selection of the work into an Increment of value during a Sprint.
The Scrum Team and its stakeholders inspect the results and adjust for the next Sprint.
The idea with running sprints is that ...
If you are using Scrum, the expectation is that the Scrum Team creates "a valuable, useful Increment every Sprint". Prior to the November 2020 Scrum Guide revisions, the term "potentially releasable" was used.
There is no "Sprint Demo" in Scrum. The event is the Sprint Review. A demonstration may be a component of a Sprint ...