As a rule, it’s probably best to avoid “random” internet sources on matters of how light works because there’s so much confidently parroted misinformation out there. For example, this is completely wrong: https://youtu.be/FAivtXJOsiI See here for correct answers to that issue: https://youtu.be/CiHN0ZWE5bk

For how mirrors work see this: https://www.scientificamerican.com/article/what-is-the-physical-proc/ https://youtu.be/rYLzxcU6ROM