project save?

i am just starting computer graphics and have no clue as to what i am doing but have learned alot from these thing i still have not figured out is if i start a project and save it then close blender ,and the next day make say a model box but i did not make it in the same project i saved before,is there a way to import the box to the saved project?or do i have to go back to the previous saved project and make the box there?

Append or link from the menu or Shift+F1. Link creates a link to the other file, meaning if you change something there it get changed everywhere you’ve linked it. Append creates a copy of it.