They don’t appear to be saying, but the investment makes sense for Apple and Google even without a sweetheart deal. It ensures a successful IPO and continuing viability of their primary chip supplier, and protects it from a takeover by some entity hostile to Apple or Google. Nvidia I’m not sure about though.
Nvidia has been selling ARM chips for years (the Nintendo Switch’s SoC, for example), and they’re currently in the process of massively growing their ARM business with the new Nvidia “Grace Superchips”. Not to mention the expected sales boom of the new Switch 2 SoC that should happen within about a year.