Using Premiere Pro cloud, so all is latest edition. I divided an 80 minute presentation into five parts. I have been researching how to merge parts together and the only method I can find is to open a project and then import another project. Note that a box to check during import titled 'image sequence' is grayed out. When I do this I find a bin with the second project's content, but no timeline (sequence?). I have found suggestions such as double clicking the second sequence with the blue line (location on the timeline) slid to the end of the existing sequence, but where is the imported sequence? It is not in the new bin.
Just use one project with as many different sequences as you need and use bins to divide up all the media and sequences in a logical way. If you already have multiple projects, the import sequences is the right thing to do. You should not check on the 'image sequence' checkbox because that is for something specific --> importing sequential images as a single clip (aka image sequence). Better, yet use the media browser to help you navigate right to the sequences you want and import them from within their parent project.