McDonald's has some incredibly rigorous requirements and processes to ensure consistent qualities in their Sprite. I have no interest in propping up a giant harmful corporation, but their Sprite really is different.
The Food Theorist and many others have made videos discussing it as well. A few months ago I saw a particular one explaining the McDonald's process at length, but unfortunately I cannot remember enough details to find the same one again.
Side note: in typing this comment and trying to find relevant videos, I had to type "Sprite" an unreasonable number of times, and not once did my phone's keyboard get it right.
One night, I went to a McDonald's expecting the fabled Sprite I've had before, and I guess their dispenser ran out the syrup. What I got was basically just soda water, and it low-key ruined my night because we got it via drive thru and I didn't take a sip until we got home 🥲
I don't know if this is true but I've read that, in their fountain sodas, McDonald's uses like 15% more flavoring syrup than what's recommended, so the sodas taste extra good
I don't know why but the feeling that pops up in my head is basically "what it feels like to be 7 years old and you have no idea about what this weird glass is but the sprite is the only thing that matters".
Honestly, that's true of all fast food places. They employ food scientists and psychologists to addict as many people as possible to their products. And once you're hooked, it's hard as fuck to get away. "I don't go to Burger King anymore, but man I miss Whoppers" is the sort of thing I've heard more than once.
Don't give money to Big Cup. Get some stone mason and jewelers tools, an emerald the size of a human heart, and DIY that cup. Save yourself at least a few bucks.