You're referring to a specific feature in the Sonic 3 Air mod, which is a popular fan-made modification for the classic Sonic the Hedgehog games. The feature you're looking for is related to the HUD (Heads-Up Display) and the Transform button.
If you’d like, I can:
Yes — some control remap mods allow binding a specific key/button to “Transform” directly, and they may add a HUD indicator. Search for “Transform Keybind” mods. sonic 3 air mod transform button hud
: This popular mod allows players to choose between Super and Hyper forms rather than being forced into Hyper once all Super Emeralds are collected. You're referring to a specific feature in the
: Brings the modern Sonic Mania look to your transformation prompts, complete with pixel-perfect icons and animations. Search for “Transform Keybind” mods
Go back to the game, enter the menu, and enable your new HUD!