The Engineering Mindset is a YouTube channel on which I've found a bunch of great lessons over the years. One of those is AC Electrical Generation Basics. While it doesn't specifically address portable generators like the one you can find in my garage, it does do a great job of clearly explaining how electricity is generated by the parts that you'll find in a portable generator.
For an explanation of how portable generators generate electricity, Doosan Portable Power offers this good explanation of one of their generators.
Can You Power a House With a Bicycle? is a video produced a few years ago by NPR's Skunk Bear. The answer is no, you can't power a house with a bicycle. The video does a great job of explaining how many bicyclists you would need in order to power an average house for a month.
Homemade Bicycle Generator was produced by Backyard Trail Builds (a channel about building bike trails and jumps). What's good about this video is that the producer explained his initial mistakes and how he corrected them. The video is also not so detailed that it doesn't inspire more curiosity about how to improve on the design.
How Do Solar Panels Work? is a TED-Ed lesson that covers the basics of what solar panels are made of and how electricity is generated from them. The video also delves into some of the political and societal barriers to solar panel installation and solar array developments.
How Do Wind Turbines Work? is a TED-ED lesson that covers the basics of how wind turbines harness the power of wind to generate electricity. The basic math of wind turbine design is also explained to viewers of the video. Overall, it's a fine lesson but not the most detailed of lessons.