How are sites created in which elements are loaded one by one?

Question

Programming languages

For example - Toaster. At first, only blocks are loaded, i.e., a kind of site frame appears, then pictures, then text, etc. How is all this done?
Facebook is also a great example.
Noticeable if internet speed is limited

Answer 1

It just takes a long time for the fonts to load.

Answer 2

I'm not sure how it works on the toaster and Facebook, but for example, this can be achieved using front-end template engines.