How do I know if activating private mode in the Quest web browser is not equivalent to sending a signal to Meta that "now extra juicy information is coming in"?

Becuase the stuff people use private browsing for is useless information for Meta.

Why do you believe that Zuckerberg wants to see your porn preferences?

You can packet sniff it if you're really concerned, no? There's been no evidence for what you suggest.

What if the packets are kept on disk and sent only when not in private mode?

What if the headset has a 4G modem built-in and sends telemetry that way? What if the next-gen Quest has a Starlink receiver in the SOC?

Ostensibly it doesn't matter. Apple's headset won't be any different, both companies will encrypt their traffic and insist it's meaningless telemetry. If these companies want to spy on you, they will - they own your hardware more than you do. Without a verifiable, open boot process you're basically just throwing stones from a glass house. If you want paranoia-tier VR computing, get a Valve Index and an airgapped Nvidia/Linux workstation. It would probably still be cheaper than Apple's complete product (somehow).

Whatever the case, I'd bet the farm on Apple's new headset shipping with OCSP telemetry out-of-box.

How do you know activating private mode in Chrome isn’t doing the same with Google?

After all that’s their business model too.

Good question, that's why I use Firefox ;)