There are two types of 5G, 5G NSA (Non standalone) and 5G SA (standalone). In addition there are a number of frequencies that 5G can run over, classed as low-band, mid-band and high-band.
5G NSA is what is most of everything that’s deployed today. This flavor of 5G requires a 4G LTE signal. If you want the details you can find them here: 5G SA vs 5G NSA deployment: What are the differences (alepo.com) The interesting things about 5G NSA is that it doesn’t require a new SIM in most cases, just a 5G capable phone.
5G SA on the other hand requires a new SIM AND the phone has to be compatible with it. The Republic SIM is NOT 5G SA compatible.
Finally, there are the frequencies, this is where the main impact is to the user.
Today, much of T-Mobiles 5G is low band (600MHz) coverage. This 5G is, well, slow. You’ll get about 4G LTE speeds. It’s more efficient for the carrier, but there’s no real impact to the customer.
Then there’s mid-band 5G. T-Mobile is rolling this out now. It will be a bit faster than 4G, not a ton, but you could potentially see double the 4G LTE speeds in some locations.
Finally, there’s mmwave or high band 5G. These are the super fast, low latency 5G speeds you’ve seen talked about. Unfortunately, this right now is more science experiment than reality. High frequency signals don’t penetrate walls, or trees, or water, or glass, etc etc. The networks that are up are quite literally line of sight, covering a few city blocks in very specific areas. Carriers are going to have to deploy many many nanocells before this network becomes meaningful. In addition, not all 5G phones are even mmwave capable. (For instance only the Verizon version of the Pixel 5 is of the Pixel line). T-Mobile basically has none of this coverage (really only Verizon does).