I am undergraduate math student who is interested in being a high school math teacher. I have been given an assignment to present to my class (for a total of about 20 minutes) a teaching tool or a teaching strategy that will enhance students’ (high school students) learning of a particular secondary mathematics topic. For example, I have a friend who is presenting https://www.canfigureit.com/, which can help students better understand and set up geometric proofs.
I don't know any websites/apps to teach mathematical concepts except for https://www.mathsisfun.com/, but this website is very generic and can be used to teach many different topics.
I was hoping that you can respond with websites/apps that you found helped high school students to discover a specific mathematical concept/topic.