More confusion left to be resolved: https://forum.xda-developers.com/galaxy-note-9/how-to/n960u-tmb-stock-odin-files-firmware-t3833205/post79410606
Speaking in terms of the United States, but the same could apply elsewhere.
Different frequency bands, and different transmission protocols (2g, 3g, LTE, etc) are used by different carriers (carriers are providers like AT&T, Verizon, or T-Mobile).
The same phone (or at least the inner radio hardware) can be programmed to work with specific carriers. Even if the phone comes from the manufacturer as different model numbers, they may have the same hardware, and are just programmed differently.
The programming is called firmware. So each carrier can have its own firmware. For instance, the Samsung Galaxy Note 9. The firmware can also be “unlocked” (which is a bad name or it, as it has nothing to do with sim-lock), where the firmware is non-carrier-specific, having the programming to work on any U.S. carrier.
Notice that the non-carrier USA version of the Note 9 is capable of many more band and protocol combinations than the other carrier specific phones. Having more bands could be useful for roaming, and for international travel, where any number of bands might be used. One downside, however, is that the non-carrier specific firmware may not work as well with a particular carrier, as the carrier-specifc firmware. I don't really know what's true at the moment, but Samsung states that the non-carrier-specific firmware works just fine on any carrier, at least based on the FAQ for the Unlocked Galaxy S9/S9+.
Each carrier has radio firmware, or baseband updates. Who makes these updates? For Samsung phones, all updates originate directly from Samsung. The carriers pass them along.
Carriers deliver updates (known as OTA updates) to your phone automatically, even to the “unlocked” phones. However, if you are using a custom ROM, the updates are up to you. Your baseband can get out of date.
Of particular curiosity to me, is if it is optimal to mix-match a newer baseband radio firmware with an older Android version.
Most people want the latest of everything. However each successive Android version runs slower than the previous, especially if moving away from the prevalent Android version when the hardware is introduced to market. Thus I am prone to want to stay with an older version of Android, but still want the latest baseband so that I have the best radio connectivity with the carrier.
Only if the group SlimRoms puts out a newer version of Android, would I trust it to run smoothly on the phone. Even then, it will not be the latest version of Android. The community needs time to customize Android into something better than the restrictiveness that Google puts out.