(Note that the second one gets into your question on how FONTS are handled in Slide Master and where to go AT THE BOTTOM of Slide Master fonts to find 'Customize'. Here are two YouTube tutorials to build your foundation knowledge and then get into advanced techniques. It is key to powerful layout setup and consistency. The topmost one is the slide master (the parent slide, so to speak).
On the left side of the screen, you’ll see the list of master slides. By default, Google Slides will select the layout master that is being used in the slide you were last in. Learning the details of SLIDE MASTER will bring tons of benefits to your PowerPoint journey. To access the master slides in Google Slides, you just need to click Slide > Edit master.
