https://devyanimations.me/am-i/
I'm using the Blocksy theme. Please compare desktop and mobile – there is a noticeable difference. I've tried Simple Lightbox, Modula, adding PHP to open images in a new tab, nothing changes the quality when zoomed in.
Single Post featured images are set to 2048×2048, but even with lower resolutions, image quality is still tanked on mobile specifically.
That is not how WordPress works, these images are too big and will slow down your site. Try cutting them in half and then for the display, try using different sizes of images, you will find it