Help with global variables and firebullet extension

I’ve been watching youtube videos about variables and global variables but they’re all based around scores, I can’t find ones that can link extensions like the fire bullet one, where I pick up an upgrade and it adds 1 stats to the fire bullet parameters like multifire, damage, etc. I want the player to be able to keep its fire bullet stats at the end of the scene and be able to carry over his current stats to the next.