to execute apps from SD
Arduino ATmega chips can not directly (natively) execute ATmega machine language instructions from SD (or from anywhere else other than the Flash program memory built in to the ATmega).
What they call an "Operating System" is based on what they call a "Virtual CPU." Similar to an "Instruction Set Emulator" or "Interpreter," as we used to call them back in the 20th century, it reads "instructions" from the SD and executes code accordingly. The set of "instructions" in the SD is not actually ATmega binary machine language, but some special stuff created by a proprietary C compiler. (Not open source, but it's very inexpensive according to the web site.)