What does an Arduino do if the serial connection is closed on the other end?
That's some strange Intel monstrosity. It's impossible to know what it does or how. Intel's foray into the Arduinosphere was brief and somewhat abortive.
In general though (for most cores) the DTR signal (and sometimes the RTS signal too) of the CDC/ACM protocol is used to &quot;gate&quot; the serial data - if it's asserted then serial data gets sent through to the host PC. If it's not asserted then it just gets thrown away.
I have seen cores where if DTR is not asserted the data gets buffered until the TX buffer is full and then blocks, but that is rare.
You might find your answer if you delve into the core's source code, but you may find that big chunks of it are hidden away in closed source pre-compiled libraries, and the fact it seems to run some custom RTOS means it'll be even more obfuscated. YMMV...