On a flight that long you’ll have 3 or occasionally 4 pilots. Even though the aircraft only needs two to fly it at any given time. Hereafter assuming 3 since that’s simpler …
Three pilots will work the first and last hour-ish, but only two will work the middle 12-ish hours for your example flight. During that cruise time they each take a turn flying and a turn sleeping in a rest spot. Which rest spot might be as nice as a real bunk bed in a quiet secluded backstage area, or as crappy as a seat in business class next to the galley with only a cloth curtain to protect the would-be sleeper from the noise & light.
So after they’re established in cruise pilot A will sleep the first 4 of 12 cruise hours, then be in the seat for 8. Pilot B will work 4, sleep 4, then work the last 4. Pilot C will work the first 8 then sleep the last 4.
As you suggest, in transoceanic cruise there’s not much to do. Over land the ATC process keeps you involved at least every few minutes. Over water there’s nothing happening for an hour at a time.
There’s the continuous background task of flying, which is really monitoring everything for unexpected changes and double-checking that you didn’t forget something or goof something the last time you touched something. But it’s far less engaging or real time than driving is. Other than you making an uncaught switch error, it’s pretty hard for something to go wrong without an alarm, a change in noise, or in motion that’d really stand out.
While you’re working reading anything or playing games on your devices are verboten. Folks converse. Tell tall tales. Argue about baseball. Union vs. Company politics is good for a couple hours of adrenaline. You can always read the company manuals. Again. Getting ever more expert on ever finer-grained minutiae.
If it’s night you can turn the lights down and see some incredible stars, aurora, etc. During the day the land and clouds can be pretty, but most of the ocean is just blue from horizon to horizon.
Many folks get good at a Zen-like state of awareness of the task at hand and little else. Becoming One with the flight if not with everything.
It’s not for everyone. Some people just aren’t that comfortable in the silence interrupted by nothing but the deafening roar of their own thoughts. They tend to fly short haul where the flying tasks keep them moderately busy from gate to gate.