60-100bpm is normal for an adult at rest. Rest means lying down resting for 10 mins.
If you have a small build and are short it is likely to be higher. Baby's often have a HR of 220 - 230 and it gradually decreases as you grow.
Max HR when exercising should be 220 - your age. So at 33 years old would be 220-33 =187.
If you run marathons your HR at rest can be very low, so low some people have to have pacemakers implanted.
Fit bits and other tech doesn't make contact with your skin the way an electrode will so I suspect all the low rates are because of the tech. What is also important is not beat to beat HR but measured over at least 30 seconds.
Other fun facts, your heart (assuming it is healthy) is about the size of a loosely made fist, it grows at the same rate.
Your HR and pulse are not the same and pulse should never be taken from a monitor or ECG, the ECG measures the electrical activity of your heart, your pulse measures the blood pumping round the body.
They are normally the same numbers but in some rare conditions the ECG can be normal with no blood being pumped.
In healthy mammals the heart beats for roughly the same number of beats over a lifetime. So a mouse has a life of about 2 years and a very fast HR, a dog - depending on size might live to 10 or 15 years and an elephants 50+ years.