This is a common misconception. If someone says they want to bring you in as a cofounder, they almost always will think of you as a significant equity partner.
That's only usual if the product is weak, your rep is high or if you have a relationship.
To be an equal you need equal equity but it is not that simple. Two friends holding 66% can dilute you, move your work into another company assgin a low value and cut you out. The many ways to get screwed is extremely high.
I got an offer from a startup as the first dev hire (CTO/Directorish) and got an absolutely laughable equity percentage. It was insultingly low. Promptly passed and thankfully broke into FAANG where I'm going to be making probably more than even if that startup had been successful with the equity split they were going to give me.
Some shiftier startups have started to use the term "founding engineer" in their recruitment materials. I doesn't mean "cofounder" at all. Rather, it's a new, confusing term for "1st engineering hire" where you get hardly any equity and still most of the responsibility and workload associated with a cofounder role.
To be an equal you need equal equity but it is not that simple. Two friends holding 66% can dilute you, move your work into another company assgin a low value and cut you out. The many ways to get screwed is extremely high.