converting assembly components into a signle component

you can save as assembly & choose part from drop down menu
when chose save assembly as part you have three option

Exterior faces
Exterior components
All components

this make deferent type file

note when you save assembly as part this part is not edit able

Use the "Join feature."

1). In the assembly model, insert a new part. Place it, exit the sketch window, but stay in the "Edit Part" window.

2). Insert -> Features -> Join.
-Pick components you want to combine. They will highlight blue.
-You can choose to "hide parts" after the feature is completed. This will
hide the components you selected to create the joined part.
-The option to "Force surface contact" will combine the components that are in contact or overlapping. Sometimes you may have an issue with "Zero thickness geometry" here, depending on your CAD, but it's nice to have the option of a multi-body part or a single-body representation of your assembly.

The advantage of this feature is that if you save it externally from the assembly model and insert it into another part/model, it will update as you make changes to the original assembly, as the "Join" feature is referring the assembly.