From what I've read, and was kinda answered here:
There is
an NHL contract drawn up for EBUGs where for one "day" of service, they get paid $500 for the game and get to keep their Jersey.
They cannot have professional experience, and cannot currently have an NHL contract.
Generally the EBUG contract gets to ride pine and never sees game action.
The home club is obligated to provide an EBUG to both home and road teams if needed.
Ayres has been called upon to be the EBUG previously with the Maple Leafs and Marlies, and was actually suited up for Carolina's AHL Charlotte Checkers against the Marlies just three weeks ago, but never saw game time.
He has his own mask (or one painted for him), but was (from what I saw by the excited former/current goaltenders I follow) using an old set of pads and gear from current Marlie Kasimir Kaskisuo, and a goalie stick from former Marlie Garett Sparks. He has occasionally filled in as netminder for both the Leafs and the Marlies over the last few years when they were in a pinch and couldn't get a goalie to practice in time.
As for the jersey he wore, each team brings a couple extras with them along with extra material and letters in case they need to make repairs or make a new jersey for a player. As F2B&G said, it was likely cobbled together, but they had plenty of time to make it from the initial injury to James Reimer to when Mrazek was injured. There were photos posted by Ayres wife that showed him proudly holding the jersey before Mrazek was hurt.
The last EBUG to get a hero's welcome like this was former Western Michigan goalie and adult rec league player
Scott Foster who played most of the third period for the Blackhawks. Foster saw seven shots and made all seven saves to preserve the win for the Hawks that night.
The only other EBUG to see ice time in the history of the NHL was Carolina Hurricanes equipment manager Jorge Alves in 2016. Alves saw the last seven seconds of game time from the net, but I believe he never faced a shot.