Isn't the buffer the 4.2 kwh between 77.8 and 82kwh or is there an additional bottom end buffer? I assume that the extra 4.2kwh are top end buffer..
No. The link above shows for the LR Non-Performance Model Y they drew 78.5kWh from the vehicle (including basically the entire buffer). That's all the pack has, period. There's nothing else. The Model Y Performance 2021 has a larger pack (closer to 81kWh available - the data is available on the above linked site).
The buffer is always located below 0%, and for Model 3 it is 4.5% of the nominal full pack (and I THINK it is 4.5% for Model Y as well - SMT would tell you and I'm sure someone has posted the data for a Model Y). So at 0% your car will be very upset, but the BMS will think that you have a bit over 3kWh left before you're done. And if you're extremely lucky you might be able to use some or all of that. I wouldn't try though (see above test for the variability).
There is no top end buffer, though if you look at one of my other posts today Tesla does apparently have a way of "squeezing" extra energy into a fixed number of rated miles when the car is brand new with a very energetic pack that exceeds their expectation of the design capacity. But even in that scenario, your rated miles will start clicking down immediately...they'll just click down more slowly (slower by 1-2%, so imperceptible) when the car is brand new. No top buffer.