If SoftBank sells shares owned by SoftBank, it is SoftBank that gets the money, not ARM. If ARM needs money (e.g., to fund the development of AI chips) it can create additional shares (in ARM) and sell those.
Right now ARM is worth >50% more than SoftBank itself. It seems odd that they are still holding 90%, and also committing so much cash for future investments.
Again, if ARM needs money, ARM needs to sell something ARM owns or to take out a loan. SoftBank's selling something SoftBank owns does not by itself increase ARM's cash even a little. (A company can create new shares in itself at any time, so one of the things ARM can sell is newly-created shares in ARM. As soon as ARM creates new ARM shares, SoftBank's ownership share in ARM goes down from 90% to less than 90% unless SoftBank buys some of those new shares.)