Merging two parts

I have this pipe and it is segmented in two pieces (red circle and arrows).
How can I make it one piece?
I could just delete the line in the red circle, but I would like to know if I can just select the two parts and join/merge them?
Could not find a solution for this.

Are the two parts solid groups or solid components? For good modeling workflow, they should be as that will prevent them from sticking to or intersecting with other objects. So, if they are not, make them so. Then you can select both, right-click and choose outer shell from the context menu to make them one piece.

Edit: this appears to be the same model as you shared in another topic. In that model these are loose geometry, not groups or components. In that case you could do a left-to-right selection to get the 24 edges and delete them. It would still be good to make these groups or components, though because the walls are thin and the ends are not closed, they won’t be solids and outer shell won’t work.