The Build configurations are sorted by MCU family. As in a family many similar settings can be used those settings are in the common sections.
The AVR architecture is an 8 bit architecture introduced by Atmel (now Microchip).
The STM32 family covers STMicroelectronics ARM Cortex-M chips.
The SAM series of MCU are the Atmel (now Microchip) ARM Cortex-M processors.