Technical Details
The ttn-esp32 library is based on IBM's LMIC library. It adds a simpler high-level API and focuses on The Things Network. Plain LoRa is not supported.
The LMIC code is a based on MCCI's Arduino LMIC library. It seems to be the most actively maintained LMIC library. The ESP32 (and ESP-IDF) specific hardware abstraction layer (HAL) is a completely independent work. The high-level API (i.e. the TheThingsNetwork class) was inspired by the official The Things Network library.
As FreeRTOS (part of ESP-IDF) provides multi-tasking (unlike the Arduino ecosystem), it makes sense that all TTN functions block until they have completed the operation even if the operation takes several seconds or minutes. It greatly simplifies the use of the library. Other tasks continue to run.
For ease of use, it was also decided to use a C++ class (instead of a bunch of C structures and functions). It better encapsulates the related operations and leads to shorter code. As a result, the callers of the library must also be written in C++.
The LMIC code mostly runs in a background task that is created when the TheThingsNetwork instance is initialized. It usually runs LMIC code when the DIO lines indicate an event or when a scheduled job is ready to be run.
The communication between the interrupt handler and the background task is implement with a queue (called background queue in the below figure). The same applies to the timer used to wait for the next scheduled job. So the background task either executes LMIC code or it waits for the next entry in the queue.
In the figure, the term app task is used for the task that calls the ttn-esp32 library, i.e. it's the caller's task. When it calls a TheThingsNetwork member function requiring several seconds (e.g. when sending a message), it will have to wait until the background task has completed its work. As part of the work, the background task might also wait for events (e.g. waiting until the RX window has expired or data is received).
Once the background task has completed the operation, it will post the status to a queue (the caller queue in the above figure). The app task implements the waiting as the waiting for the next element in the caller queue.
The ttn-esp32 library calls LMIC functions to start an operation directly from the app task. Because it's a different task from the background task, which is supposed to execute the operation, it needs to wake it up. This is achieved by putting an element into the backgroud task.
For AES encryption, the ESP32's dedicated hardware unit is used. Given the small amount of data transferred, the speed gain is negligible. However, the code size shrinks by about 5 kBytes.
The Arduino LMIC library can optionally poll the SX127x status register in the run loop, i.e. many times a second. Even though this is not optimal, it can work without wiring the DIOx pins. The ttn-esp32 library does not implement polling and requires the DIO0 and DIO1 pins to be connected to the ESP32.