> Just on my phone, several apps let me subscribe via the Apple AppStore but also by subscribing on their website with a credit card (like Discord, NYT, AllTrails, HBO, etc)

This is one of the few historical exemptions from Apple's otherwise ironclad IAP rules for so-called "reader apps".

Still, until very recently, these apps could not even mention that you can buy content/subscriptions on their website for cheaper (or, in some cases like the Kindle app, exclusively buy it there), let alone link to said website.

Now, the landscape is somewhat fragmented: In the EU, thanks to the DMA, external linking to external payment methods is allowed, but comes with a "scare sheet" and still grants Apple a cut of 20%(!). (This is about to change, I believe, but I'm not super up to date on it.)

In the US, developers can now apparently link to external payment sites (in SAfari) without Apple being allowed to charge a commission at all. This is a very recent development, and I haven't seen it in action myself.