Hello OP I’m Muslim and have a degree in Theology, so will try to answer this from an Islamic perspective and hope it can answer your question.
For most people, Islam is a religion that started in the 7th century in Arabia, and its ‘founder’ is Muhammad. That’s might seem like a simple, neutral, objective statement but it’s something that Islam itself doesn’t teach.
Traditionally, Muslims believe that Islam is the primordial religion of all time and all peoples. Islam itself means submission to God; therefore, anyone who submits to God whether they lived before or after Muhammad would be considered a Muslim.
Islam also teaches that every nation on earth received a Prophet/Messenger from God and the message of each of these Prophets was the same in terms of worshipping one God, leading a moral life, etc. Thats why Muslims believe not only Jesus, but Abraham, Moses, John the Baptist, etc were all Muslims. That’s why it’s possible for Prophets to exist who were Jewish in terms of ethnicity, culture, etc but Muslim in their religious outlook.
Some Muslims also believe that during the lifetime of the these prophets, the Shahadah or Deceleration of Faith would have been ‘There is no God but God and Moses/Jesus/Muhammad etc is the Messenger of God.’
It’s why Muslims also state that all humans are born with a natural disposition towards this act of submission to God. It’s why a lot of people who become Muslim in later life prefer to say they have reverted as opposed to converted.