Shares in the U.S. markets are held by a Custodian (ETC) in a segregated account. DriveWealth, a FINRA regulated broker dealer manages the books and records of the securities and cash.
Settlement is T + 2 business days from when the trade is placed. Execution is on the National Best Bid and Offer system. Your account is insured for up to US $500,000 under the Securities Investor Protection Corporation (SIPC) insurance. This setup is similar to all other brokers who provide trading services into the US markets.
If you are on Stake Black however, you are able to trade on unsettled funds. Learn more here.