When you have two or more flights one after the other and if you have booked these flight separately, it’s called a self transfer.
If you have two or more reservation numbers, it’s a self transfer.
If you paid for these flights separately, it’s definitely a self transfer.
Sometimes it might be hard to distinguish the two, self transfer and airline transfer. For example, some providers allow you buy several flights as a single booking. But if you look at the boarding passes, you’ll see that each one of them has their own reservation number or so called booking reference. That means it’s a self transfer.
Mostly flight search sites tell it somewhere, if it’s a self transfer.
When you buy flights from several airlines, it's going to be a self transfer.