4 Year Old Asking Where Babies Come From?

My 4 year old son has recently been asking me where babies come from. Pointing at old pregnancy photos of me and asking how a baby got in my belly and that type of thing. I was shocked he wanted to know that at this young of age. What is appropriate to disclose and what’s not ? I have two children but he is my oldest so I’ve never been through this before. 😩 Please help!