How to combine different projects to create final clip in After Effects?
First, you don't want to render the clips prior to final output if you want maximum quality.  You instead want to use nested compositions.  Really, final assembly is ideally done using an actual non-linear editor, such as Premiere, however you can accomplish the same thing by putting together a composition and using each of the compositions you have made as assets.  Then, when you output the final render, it will render what it needs from each of the nested compositions and keep the highest possible quality.

Note that you should use nested compositions regardless of if you use Premiere or After Effects.  Premiere will import compositions directly from After Effects if you import the After Effects project in Premiere.  You may also have to apply scaling in order to get the clips to match resolution.