I was an aficionado at collecting links from websites with math java applets that allowed interaction to learn mathematical concepts visually and interactively. My favorite was Manipula Math.
But since Java is running out of support in the current internet browsers and has been replaced by HTML5 my links do not work anymore.
Therefore, I have proposed to list in the answers to this question a collection of websites that contain applets in HTML5 format that explain mathematical concepts interactively.
To start this list I indicate the sites:
https://www.mathwarehouse.com/interactive/html5-applets/
https://www.geogebra.org/t/functions
https://www.geogebra.org/t/algebra
https://www.geogebra.org/m/aHUtYYRK
https://www.geogebra.org/t/calculus