If I were a billionaire, I would start a non-profit and build tiny home communities all across the US to end homelessness. Each community would provide mental healthcare, education services, healthcare (including dental and vision), to meet people where they are and help them thrive.
Mostly I would just spend it/ give it away. 10% for myself and my family would be life changing for us all and $900,000,000 would do so much for those who needed.