Does the October 15th tax deadline apply to everyone who filed an extension?

i'm a ittle confused about this whole extension thing. I filed one back in April because I knew I wasn’t gonna be ready, and now everyone keeps talking about the October 15th tax deadline. Is that the hard deadline for everyone who got an extension or are there exceptions. I don’t wanna assume I have more time if I really don’t. The IRS doesn’t exactly send reminders like hey don’t forget you’re almost out of time. For anyone who’s been through this before, did you have to be 100% filed by October 15th or did they give wiggle room. I just don’t want to get slammed with more penalties for misunderstanding the rules