Do stake pool operators earn a percentage on the ADA delegated to their pools or does the amount of ADA delegated has no bearing on the earnings of stake pool operators?

There is a fixed fee at the moment of 340 ADA per epoch universally imposed by the protocol parameter. This is paid from eligible pot reward prior distribution to delegators of that specific pool. Once that is deducted - then % margin is removed. E.g. if Pool has 5% margin fee - 95% of all rewards go to delegators and their ADA proportional to their holdings in that pool.