https://webapps.dol.gov/elaws/whd/flsa/docs/volunteers.asp "Under the FLSA, employees may not volunteer services to for-profit private sector employers."

https://www.shrm.org/resourcesandtools/tools-and-samples/how... "There are no general regulations that permit volunteering of services to an employer in the private sector. All hours worked must be paid. According to the FLSA, an employer must pay all employees not less than the minimum wage for all hours worked."

https://hrdailyadvisor.blr.com/2018/01/04/can-accept-volunte... "Most for-profit organizations cannot accept volunteer, unpaid labor without running afoul of the FLSA."