It was not until a few hundred years after his disappearance that several schools appeared, variously interpreting his doctrine. For some, the main thing is the Principle of Buddhahood which is embodied in many Buddhas in the past and the future, and not just the historical Buddha. It is the Dharma Body (dharmakaya in Sanskrit), the transcendental Buddha who becomes timeless and eternal, identical to the “Buddha nature” (or “Buddha embryo”, tathagatagarbha in Sanskrit) present in all living beings.
It is thus with the development of the Great Vehicle in the northwest of India, then in central and eastern Asia, that Buddhism became a true religion, with the cult of Buddha and Bodhisattva. Various religious rites, non-existent in the time of the Buddha, were created and used as "skillful means" to achieve enlightenment.
“Follow the Buddha's teaching if you want, but keep your religion”. The Dalai Lama and Master Thich Nhât Hanh
Among the schools of the Great Vehicle, only Chán (or Zen) remains essentially a philosophy of life, based on “emptiness” and letting go. Thus, to the question: "Can one be both a Buddhist and follow another religion?" », I would answer yes, if one considers Buddhism as a philosophical doctrine and not a religion. Moreover, during his lifetime, the Buddha always showed great tolerance towards other philosophies and religions. In particular, he advised followers of Brahmanism who wanted to follow him to continue to honor their master and their religion.
This is also what the Dalai Lama and Master Thich Nhât Hanh advise people who turn away from their religion to go to Buddhism: “Follow the teachings of the Buddha if you want, but keep your religion”.
Are there points in common with the three main monotheistic religions, Judaism, Christianity and Islam?
Yes, of course, there are commonalities between Buddhism and these three main religions. It is essentially about ethics, like the Five Precepts fundamentals of Buddhism: "not to kill, not to steal, not to lie, not to commit illicit sexual relations, not to get drunk or take drugs", which are very close to the Ten Commandmentss of God, transmitted to Moses.
Like all religions, Buddhism advocates peace, love of neighbor, brotherhood and help to the most vulnerable. The only difference is that it is not in the name of God that he does this, but thanks to the deep understanding of the Principle of Dependent Origination in the universe.
We also find in all these religions similar practices, such as meditation, prayer, the study of the scriptures, giving, helping the poorest, charitable or compassionate works.