Apple Pay is supported by Stripe, so it's a non-competitor (Apple Pay is just a way of giving your card details to a processor).
PayPal is the closest competitor, but their APIs are quite old (although improving) and when Stripe launched PayPal's pricing was very cmoplex. Also people just don't like the PayPal model for business.
Square is a payment processor, mostly, for physical payments mostly seemingly for hipster coffee shops. They do online payments, but nowhere near the same level Stripe do them.
I asked my professional dev friend what the heck was going on and he told me to use Stripe. Never heard of Stripe before that. Friend also told me his revenue was frozen for weeks/months by PayPal for his projects. A weekend later, I was done implementing Stripe.
My partner insisted I use PayPal because that's what everyone recognized/used, but I flat out refused after implementing Stripe.
The feeling is the same for most "professional" developers too. The old XML API is horrible by modern standards.
They do have a new one though that's less of a pain.
Stripe can make that little box you type your credit card painless, seamless and highly accessible for small and large companies. They are generally delivering a high quality experience throughout their process and many of their staff are generally awesome.
Full disclosure, I work at a big company and we use Stripe. I used to work at startups in the early 2000s and handled payment solutions. The world is better with Stripe.
Is Stripe popular because its developer friendly? What makes it better than Apple Pay, PayPal, and Square? Normally, when I shop online, I’m usually given an option to type my credit card info or use my PayPal account. Where does Stripe fit into all of this?