Isn't "to" necessary between "helped" and "found"?
You can place "to" between "helped" and "found" but no, "to" is not necessary for this sentence. "Helped is the verb of the sentence. This verb can be used with the infinitive "to" and also without the infinitive "to." You may already know this but the infinitive helps modify the verb. In this case, if we said "to found" it would modify the verb "help." The sentence could be "He helped to found the company that became PayPal." This phrase is a little more formal but in spoken English, it's more common to say "He helped found the company PayPal." Here are example sentences where "helped" is being used with and without "to." Ex: She helped to discover the dinosaur fossils. = She helped discover the dinosaur fossils. Ex: He helped to decorate the gymnasium. = He helped decorate the gymnasium.