With about 305 million active users, PayPal is perhaps the most popular and largest payment gateway. Do you need someone to send you money? PayPal. Do you need a payment gateway so you can pay for your Netflix or Spotify subscription? PayPal. Was your WSO rejected for not having a valid checkout link? PayPal.
Stripe is akin to PayPal in the sense that it's very easy to use and requires no membership or any other fees. Stripe, however, is a customized payment platform with special tools and features any developer can use to create a custom payment platform for any business.
Amazon Pay is, well, Amazon's in-house payment service to quickly and easily reach out to any users within the platform. This can also be used to make donations and payments for any good or services on third-party websites and is integrated with Alexa.