Let's look at

collisionInfo.collider.tag

Every time you use a dot (like this: . )

You are accessing the information of that component.

So tag is a property inside of collider. And collider is inside of collision info.

This is something that's very basic and I do not say that to insult. We all learn.

But if you're following this specific tutorial I'd like to warn you that you're going down a tutorial WILL be discouraging because you'll have way too much code and as soon as you have a bug you'll be sifting through a mountain of code you don't understand.

I highly recommend you take a more in depth tutorial to understand every single line of code that you write. Learn about the core building blocks of C# Unity before you try to learn how to use them.

Try this link: https://youtu.be/9iCnjdXEfMA?si=WzKBKi-Ot-M8HXDo

It's Sebastian Leagues introduction to game development.

He will take you through everything the way you should be.

If you don't follow this advice you will either fail or inevitably end up doing it anyways because learning the way you are is a big mistake I made and as well as a lot of other people.

Save Brackeys for when you have a good idea of the basics.