Product Owner - Do you know how to build/update the backlog?

The Product Owner is responsible for owning the Product backlog. See the video on "Do you know how to be a good Product Owner?"

How to add new PBI (Product Backlog item) to the backlog?

Do not use emails as you can't order them by the business priorities.

Use Azure DevOps (E.g. https://ssw.visualstudio.com) as it allows you to enter an item into the backlog, in the preferred priority order.

Note: You can also create a PBI using Azure DevOps and attach an email directly if needed, without using Team Companion