first of all here's a Wikipedia article on it
https://en.wikipedia.org/wiki/Image_scaling

Second of all my advice is I would probably just design the avatar with 150 by 150 in mind but if that's not an option this can hopefully help

And third of all if you're having issues with blurry art you can try nearest neighbor but it can also have its issues with jagged edges so pick you poison you also have other scaling methods like Lanczos

But the two I'm going to briefly explain is by bicubic and nearest neighbor

So first of all I'll start with the 1920 by 1920 picture this is just my comic fury avatar and I also use it on other sites


So the most common scaling method and the one you probably used is bicubic In which it blurs the pixels this is an example I've downscaled it using bicubic then upscaled it using nearest neighbor so you can see the pixels better


The second one which is nearest neighbor you get really jagged edges this is more better for pixel art but you figure out what's best