Currently, in a next-gen project you can not edit the filter for reading issues from different projects. The backlog only reads from the selected / working project.
We have several projects created in Jira Software Cloud (classic projects) and have a project called SCRUM where we create a board for each SCRUM team. Each board reads from several projects an the P.O. organize the backlog of each of them.
As I have seen, this can't be done in next-gen projects.