The Samsung Galaxy S21 is continuing a trend that other 5G phones before it already started: It's compatible with all 5G networks from all US carriers, and it even supports some technologies that are yet to be deployed. You won't have to worry about carrier interoperability when you buy an unlocked S21 in the US — they work on all carriers.
The extensive 5G support is enabled thanks to the Snapdragon 888, Qualcomm's latest high-end processor equipped with the latest 5G antennas. The integrated X60 modem is also the first to broadly support carrier aggregation between different kinds of 5G in the US. That might not sound very exciting, but it means that phones with the 888 can connect to multiple 5G frequencies at once, bundling signals to potentially achieve the super-fast connections we're all waiting for. Carrier aggregation already increased 4G speeds over the last few years, so it's great to see the technology becoming standard for 5G. Earlier Qualcomm modems only had very limited support for carrier aggregation.
The S21 is also certified to work with the new C-band, the upcoming 5G frequency that lives between the 2.4 and 5Ghz Wi-Fi bands. This spectrum isn't as crowded as others and was originally reserved for satellite television, but some of the lower band spectrum could be freed up for mobile internet thanks to technological advances. The inclusion of C-band support will help with future-proofing, which is extremely valuable if you're someone who only upgrades their phone every other year.
Other than that, the S21 supports all the bands and connection methods you might have heard about already: There's 5G Non-Standalone (NSA), which falls back on some 4G technology for control and signaling functions; 5G Standalone (SA), which uses 5G for both signaling and information transfer; mmWave, the ultra-fast short wave technology with an extremely short range Verizon is pushing; and Sub-6, which has better coverage than mmWave but can't reach its speeds.
It also deserves stressing that the S21 doesn't only hit all of the 5G bands — it additionally supports virtually all 4G networks and bands currently in use in the country, making it as universal as it can get in the US market.
The world looked different when 4G first became a thing. Many 4G phones had to be (and many cheaper handsets still have to be) purchased for a specific carrier, as each required different bands. Including antennas for all networks was too expensive in the beginning. Luckily, that is mostly a thing of the past with 5G, with the exception of mmWave — the expensive technology requires a huge antenna and is often omitted in cheaper phones, like the Pixel 4a 5G.