As new user I can’t even upload my stuff (file), so there you go 
Did you move the object with translate tool? Or manually?
Manually should i use that tool? Iam quite new so i dont use many tools
The objects must be touching each other in order to unite. Translate tool would insure they are touching.
An easier method to create your handle/knob could be to select the top surface of your lid and sketch on it to create an extrusion from it.
Any surface can be your next part’s starting point,
you can group them by selecting what ever needed, then create new folder if they don’t touch each other , even make under folders in a folder. only issue if you use that way you need to name those folders correctly so you can directly select by clicking on the specific folder than move on from there.