— The spec says it inserts the children of the fragment, and that's what they do.
— It also says those functions return the inserted nodes.
— Check mdn, I updated the docs
— Https://developer.mozilla.org/en-US/docs/Web/API/Node/appendChild
— Https://developer.mozilla.org/en-US/docs/Web/API/Node/insertBefore
— This has everything you need to know in source
— DOM living standard says very little nowadays and is honestly a not a good document.