What do you mean by multi-tenant apps? I hear multi-tenant in the context of hosting infrastructure, so EC2/EKS/Heroku would be mutli-tenant. But a multitenant app, wouldn't that be any app? Like say, stripe or github?

Yeah, stripe, github, slack, etc. are multi-tenant apps - they run the same system but each company's data is supposed to stay separate. EC2/Heroku are infrastructure multi-tenants, so they isolate at vm or container level. App multi-tenancy - isolation happens at the app and db level.