They’re not owned by T-Mobile, but they’re just an MVNO.
Everyone can save by switching to an MVNO. The first trade off is that the big 3 carriers have essentially a tier list of customers based on what type of service they have for network priority. E.g., postpaid customers are highest priority, big carrier prepaid and big carrier-owned MVNOs are second, and unaffiliated MVNOs are placed based on the specifics of their agreements with the carrier. For example, Google Fi has high network priority because they undoubtedly pay more to use their networks than lower cost MVNOs.
The second trade off is that MVNOs handle customer service and a lot of the customer facing administration. This can vary but is often pretty barebones for lower cost MVNOs.