Milk is really common in recipes. Almost any liquid can be used as a substitute depending on your specific dietary needs and/or desired result for a recipe. When our kid's needs were dairy-free the only safe substitute we had available was water. Most of the time substituting half the amount of water for milk worked just fine. At times it affected the consistency or baking time, so we would play with varying the amount. Being able to use some of the more common substitutions can modify the taste of a recipe & may need to be tweaked for your palate. If tweaking doesn't help, try a different ingredient. Many recipes on this blog have been done successfully without milk.
Common 1:1 substitutions for milk:
Water - can do 1:1, but for most we found doing 1/2 worked best
Coconut milk - the one we've used is thick so we mixed it 1/2 & 1/2 with water
Toasted Quinoa milk
Alternative milks can be store-bought or homemade. You will still want to read labels of store-boughts depending on your specific needs. We even needed to contact the manufactures to find out unlisted ingredients because our kiddo was still reacting to ones that they shouldn't have been. Precede cautiously & when in doubt do not use it.