How such an agreement was formed is unknown.
Apple has been developing its own platforms for smartphones for a long time, and for the last few years for PCs. These SoCs are built on Apple's own CPU, but are based on Arm architecture. Apple reportedly pays Arm less than 30 cents for each chip it produces.
It's unclear what the normal price is, but Apple is reported to pay less than all other licensees. In addition, this same amount (less than 30 cents per chip) is paid for any Apple SoC, regardless of its characteristics. Because of all this, although the Cupertino giant is one of Arm's largest customers, the latter receives less than 5% of its total revenue from Apple.
Back in 2017, SoftBank's CEO gathered a group of executives from Arm, which SoftBank had just acquired at the time, to complain about Apple. At the time, he said Apple was paying more for the piece of plastic that protects the screens of the new iPhones than for the Arm license. What the amount was at that time is unknown, but the current 30 cents (more precisely, less than 30 cents) clearly indicate that if SoftBank tried to change something through negotiations, it did not work out.