GeoJSON is what web mapping libraries and GIS tooling produce, but Google Earth and Google My Maps want KML. This rewrites your features as KML placemarks, keeping points, lines, polygons and their holes, along with whatever name and description each feature carries, so the result opens directly in Earth without a round trip through desktop GIS.
The direction that usually gets skipped
Almost every converter handles KML to GeoJSON, because that is the direction developers need when they pull data off Google's platforms and into a web map. The reverse gets far less attention, and yet it is the one non-technical people hit: someone has been sent a .geojson file and simply wants to look at it on a map they already know how to use.
Google Earth will not open GeoJSON. Neither will My Maps import it. The file is not corrupt and nothing has gone wrong, it is just the wrong dialect for that particular reader, and this converts between the two without asking you to install QGIS to do it.
What survives the trip, and what cannot
Geometry survives completely. Points become placemarks, linestrings become paths, polygons become polygons, and interior rings become proper innerBoundaryIs elements rather than being flattened away, so a field with a lake in the middle still has a hole in it. Elevation is preserved where your coordinates carry a third value.
Properties fare less well, because the two formats disagree about what a property is. KML gives a placemark one name and one description, whereas a GeoJSON feature can hold arbitrarily many attributes. The name and description are mapped across; anything else in the properties object has no obvious home in KML and is not invented one. If those extra attributes matter, keep the GeoJSON as your source of truth and treat the KML as a view of it.