How to delay the loading and rendering of selected assets

How can I delay the loading and rendering of selected assets beyond the initial rendering of the loaded page. With Assets I primarily mean:
1) JSON (Lottie) files
2) Images
3) Self hosted Videofiles
I do not mean a general “Lazy loading” of a specific file type. I mean really selecting a specific file and loading and rendering it after the rest of the page has been rendered in the browser.

And is there a way to easily set “Fallback”-Versions to load first (with all other content) and then be exchanged after the corresponding big assets have loaded.