How the heck did my cat get worms ?!?!

*** i am not seeking medical advice. I have already spoken with a few vets and will be taking all three cats to the vet in a couple weeks. *** Just for a small backstory on my kitties. I have three of them. The oldest is about 8 years old, middle child is about 4.5 years old and our youngest is about 9 months old ( brought him home about 7/8 months ago) I guess this might be more of a rant than anything else but any insight would be appreciated. So my cat has tapeworms. The thing is, the other two do not have them as far as i can tell. I've searched and searched and searched through their poops and their bootys for a solid week and there is absolutely no evidence that my other two are infected. The other thing is i can't find any evidence of fleas, besides my middle child cat having worms. I'm washing every single piece of cloth in this house and will be giving all of them flea baths just in case but i am wildly confused at how this could've happened. They are solely indoors, except the one who is infected will very occasionally go outside for supervised outside time and he just likes to lay on the concrete, and only for 5/10 minutes at a time. I'm just so confused. Could he have gotten a few fleas on him, eaten them and that's how he contracted the worms ???