Hello @fjleon
Each Google/Nest WiFi (and Nest WiFi Pro) unit dynamically chooses a WiFi channel in the 2.4GHz band based on measured usage in the vicinity. But, in the 5GHz band, there aren't actually that many to choose from due to the DFS rules for much of that band and the wider channel width used here, so it's statically configured into the units based on their region (in the US, they use a channel in the upper part of the 5GHz band, but in the EU, for example, they can only use a channel in the lower part of the band).
Nest WiFi Pro uses the 6GHz band to serve clients as well as carry the mesh interconnect traffic, but there isn't as much congestion in that band (yet), and there are more channels available to start with.
Given the limited number of 5GHz channels that are practically available, it's not a surprise your T-Mobile system is using the same 5GHz channel. That doesn't mean it's a big problem, though. I would try to ensure the two radios are at least 3 feet apart, but as long as nothing is actually connecting to the T-Mobile WiFi, it will just be transmitting a few beacons each second, so it shouldn't cause any actual congestion.
