Different companies, but offering competing services.
If you have an app, you can host with AWS, Azure, Digital Ocean or Heroku or Google App Engine.
They offer different tools, setup, supported frameworks, configuration options, and pricing also.
Although Heroku is hosted on top of AWS, but AWS, Azure, Digital Ocean, and Google all have their own datacenters (or rent datacenters).
There are comparison tables out there, but generally there is usually a trade off for easiness of setup, how much dev ops skills you have vs. flexibility and price (some price scheme works out better if you are small some price scheme works out better if you are big).