Themes includes the designs and stylings while plugins provide the functionality like for example in this i added the theme of mordern clinic so it just provided me with the overall design while i intalled the elemontor plugin which gave me the tools to add more functionality.

Child themes:

so child theme is basicaally a folder on top of the main or parent theme in which you add customization for your styles or themes or even add functionality. you can also add to the main theme but whenever that theme will be updated your whole customize code will be deleted so to overcome this problem we made child themes