Given that the 2026 Games will be a far smaller affair than 2014 as noted by
@TheGrandWazoo, it's highly unlikely we'll see as many shuttle buses in use or as large a network of services this time around. There will undoubtedly be shuttle services, I don't doubt that. Who will operate them, how frequent they'll be, what routes will be taken and the destinations served (not counting the event venues obviously) aren't yet known, but I'm sure more information will come out the closer the start date gets.
IIRC, the 2014 games enjoyed some brand new e400s that First sent up there in advance of them heading to Manchester (?) as well as some hired-in fleet (Translink?). That said, the 2026 games are much smaller than 2014, with fewer venues (only 4) and there will be no athletes village.
The 2014 shuttle services were supplied by, barring the First stock in the Games Transport fleet (a mix of E200s for Cymru, E400s for West Yorkshire & Manchester & Streetlites for Aberdeen & Scotland East, topped up by loans from the main Glasgow fleet), by the following operators (after a quick Flickr search):
Stagecoach (which ran most of their vehicles under the Western license with vehicles drafted in from across the empire, including a motley collection of ex-London ALX400 Tridents), National Express (these running under the Dundee license), Reading Buses, Translink/Ulsterbus, Go North East (all of whom hired vehicles out to FGT iirc) & McGill's (who put their ex-London Citaro artics to good use on various shuttles to/from the city).
If memory serves (could be totally wrong here and it's likely a false memory), Stagecoach had a temporary depot somewhere in Glasgow for their Commonwealth work, plus they stored vehicles at the GVVT garage at Bridgeton, while all the others barring McGill's were stored at First's brand-new Gushetfaulds Depot (as Caledonia was then known as; Larkfield wouldn't close until October that same year).
