I don't understand bodhicitta

I have difficulty with the notion of bodhicitta and the concept of Boddhisattva, and the difference with the concept of Buddha. If I understand correctly, the idea is to have an altruistic intention in one's practice: to seek liberation not for oneself, but for all sentient beings. I can understand the usefulness of having an altruistic intention as a basis for one's practice, but I have the impression that for Mahayana this must be taken as a real objective: however, it is held in Buddhism that samsara has no beginning and no end, therefore neither does ignorance, and that there will never be a moment when all beings will be liberated. In the same way, the practitioner who attains enlightenment obtains it only for himself, and cannot give it to others like a deity who would offer a grace: "each one is his own refuge". So what is the real scope of bodhicitta? Is it only a pious wish? How can one truly believe that one is really practicing for the liberation of the world and not only one's own, if one knows for a fact that awakening is always "personal" and that samsasra will always exist? If the idea is only to awaken in order to be able to teach others, this is what a Buddha does and there is no need for the concept of Boddhisattva for that. If the idea is to help others and to have a mind radiating compassion, this is also what a Buddha does with the brahmavihara and I don't see the difference with the Bodhisattva.