Why do some characters are not displaying correctly in this website?
You presumably need to download and add a font.  Try Symbola.

If you have an iOS device, I think you should find the same behavior.  Apple's supplied fonts do not cover all of Unicode, which has well over 100,000 characters.

To check whether your MacOS has a font covering a specific character, you can open Character Viewer (Edit > Emoji and Symbols), select Unicode, and find the hex number.