PS Hang on, though, What is SBAS? | EGNOS Portal says WAAS and EGNOS are interoperable and a receiver which can get one can get the other; and Receiver list | EGNOS Portal lists the Aera series as compliant. Confused again...
That's correct. WAAS capable devices can take advantage of EGNOS. If your Garmin shows 3D Differential, it makes use of EGNOS. You can check the satellites it tracks, the ones with numbers 33-51 are the EGNOS satellites. There are a few geostationary satellites that receive correction data from ground stations in the UK and France (+ Spain for Artemis) and broadcast it on the same frequency as the standard GPS satellites.
Differential GPS (DGPS) was previously used (only in the US I think) and it consists of fixed ground stations that know their exact position and determine their GPS position. The difference is then encoded in a long wave (LW) signal that travels over the ground and can be received by other stations to improve their position fix. The idea was to install such ground stations in the vicinity of aerodromes but having a few space vehicles providing more generic correction information gets us to ILS accuracy as well and is of course much more convenient.