You're confusing 2 different things here. SBAS gets you (roughly) from 5m to 1m, to meet requirements for aviation use. RTK gets you from 5m to <0.1m. Both use base stations to monitor deviations, but for RTK the base station is intended to be <100km from you and the data needs to have as little delay as possible. SBAS base stations are a few per continent & don't care about delay to the same degree.
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I believe the various networks that supplement GPS/GNSS systems are called SBAS
(the USA has WAAS for aircraft)
and the fantastic opensource GPSTest app will show you what your device supports
https://github.com/barbeau/gpstest
You're confusing 2 different things here. SBAS gets you (roughly) from 5m to 1m, to meet requirements for aviation use. RTK gets you from 5m to <0.1m. Both use base stations to monitor deviations, but for RTK the base station is intended to be <100km from you and the data needs to have as little delay as possible. SBAS base stations are a few per continent & don't care about delay to the same degree.
The SB in SBAS stands for satellite based. RTK is ground station based.