Do you have any links or more info about the special internal mechanism? Would need an apple employee to initiate this I assume?

"When you give money to an eligible organization, we’ll match your donations one-for-one, so your $1 has the impact of $2. And if you choose to donate your time, we’ll contribute $25 for every hour you volunteer. Whether you donate time or money, Apple will match your contributions up to $10,000 a year."

https://www.apple.com/careers/us/life-at-apple/benefits.html

That is neat- any idea what they consider an eligible organization?

They're probably using something like https://benevity.com/ to do the vetting, esp. for non US charities.

Any non-profit, or just charitable non-profits (aka 501(c)(3))? Unfortunately, the US does not consider producing open source software to be charitable activity.

It can. There are a number of charitable foundations that support open aource. For example the Python Foundation.

But it is on a case by case basis, and it does take work to get the IRS to accept it.

Interesting. In any case, libsodium's fiscal sponsor (Open Source Collective - https://opencollective.com/opensource) is not a 501(c)(3) non-profit.