In my experience the tachycardia has a cadence through the day as the resting HR wanders slowly up and down, with superimposed spikes with standing, moving, eating etc. PEM would likely up both the resting and maximum HR. In the moment, I don't think overdoing it cognitively would likely reflect an early change in HR, though may well show as a delayed effect. Others' experience may be different however.
Not unreasonable given what we've learned with Cambridge Analytica, Snowden etc. He might favour Linux or similar open source platforms in general, as they put control in the users' hands. But while Android itself is Linux-based, I would regard the app ecosystem as a bit of a "Wild West".
While certainly not open source in the majority, counter-intuitively you might be better with the Apple platform. Technically it's high quality, although you do pay extra for that; but most importantly Apple takes protection of user data very seriously and it's a point of distinction. See their legal fights with the FBI/DOJ for example. This isn't always to their advantage, eg Siri is technically very behind other voice assistants, partly because Apple keeps user voice data on-device where Google et al have it all on their servers.
They take particular care with personal health and location data. Although it's backed up in iCloud by default, Apple themselves do not have the keys to read it.* Individual apps can of course sync the data you give them access to to their own servers should they wish, but many iOS app makers follow Apple's lead and keep your data on-device and under your control as much as possible.
This difference is part of the cost differential - as many see it you're paying full price to Apple, as opposed to half price to another manufacturer with you and your data making up the residual. Or the oft-spoke Facebook/Twitter line: "if you're not paying for the product, you are the product."
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* Even if the FBI asks nicely. See the table at
https://support.apple.com/en-au/HT202303