Is there card holding slots inside?

There are 3 pockets, no holders inside. Could have been a really handy bag but after only using 3 times the shoulder strap started to fray & then split.

My phone with folio case (iPhone 7 plus) is 6.4 ins x about 3.7 ins. Would this fit in bag and zipper be able to close.

I think it'd be OK height-wise (just) but a bit of a tight squeeze width-wise in the largest of the three pockets. It's a bit of an imprecise measurement, but I've measured the material part of the largest pocket and it's about 3.7 ins, so you wouldn't have much room to play with. Hope that helps :-)

Is the strap long enough to wear across the body?

If you are skinny, however I am chunky so It is a little tight and uncomfortable

Would an Apple 8X fit?

I am not familiar with this type of phone, but i have a samsung which is a fairly large phone and fits in easily my phone is 6 inches..bag is 7inches and the compartment is fairly wide...you could fit a phone in 6 1/2 inches...i hope this is of some help..i love it haven't used a handbag since...i can fit glasses lipstick, loose change, my mask ...