I'm currently using the HTML editor to create my website. I'm going to have video logs as a big part of the website. I want to add them at a later time, but I want to have them added as thumbnails, that, when clicked, will automatically play in full screen. I'm having a hard time figuring out how to create the HTML code to do that. Any assistance in this matter will be greatly appreciated.

Have the thumbnails open a "REVEAL" which is set to "FULL height & width (100% each or 100vh 100vw , respectively whichever you prefer) animate the reveal as you wish (fade in, fade out, slide in, slide out etc ) you can even nest reveal containers and go from 1 to another by including customised controls.

There are plenty of examples online some with free code & stuff W3schools also have tutorial for building a reveal and customising it.

Or do a Zoom on hover tp enlarge the thumbnail (not as nice as the reveal though)