What is the best way to pass login details to another server/website

I have multiple SaaS (Software as a Service) Platforms which I now want to connect.

The goal is that the users signs up on Platform 1 and pay there, then they can click on "Go to Service XY" and get redirect + logged in the selected service (the service is on another domain + server). Vice versa... So what is the most secure way?

The Main Application is written in react and the others are different PHP frameworks...

Best Regards