Strangler pattern is a way of migrating from legacy applications to new ones with both being operational at the same time and the upgrade happening behind the scenes gradually.
In a multi-tenant environment the application database is usually partitioned by tenants. This is done to achieve isolation and scalability. The problem of course is how do you route tenant requests to the correct databases?
Application services can be loosely compared with the business logic layer of the yesteryear with one key difference, application services don’t actually contain any business logic neither they enforce any business logic.