How are variables stored in and retrieved from the program stack?

Apologies in advance for the naivety of this question. I am a 50 year old artist trying to properly understand computers really for the first time. So here goes. I have been trying to understand how data types and variables are handled by a compiler (in a very general sense, I know there is a lot to it). I'm missing something in my understanding of the relationship between storage in "the stack" and value types, and storage on "the heap" and reference types (the quotation marks are meant to signify that I understand that these terms are abstractions and not to be taken too literally in such a simplified context as the way I am framing this question). Anyway, my simplistic idea is that types like Booleans and integers go on "the stack" because they can, because they are known entities in terms of storage space, and their scope is easily controlled accordingly. But what I don't get is how variables on the stack are then read by an application- if I declare and assign x as an integer, say x = 3, and storage is reserved on the stack and then its value of 3 is stored there, and then in the same function I declare and assign y as, say 4, and then following that I then use x in another expression, (say z = 5 + x ) how can the program read x in order to evaluate z when it is below y on the stack? I am clearly missing something. Is it that the location on the stack is only about the lifetime/ scope of the variable, and that the whole stack is actually accessible to the program all the time? If so, does that imply there is some other index that holds the addresses only of the variables on the stack to allow the values to be retrieved? But then I thought the whole point of the stack was that values were stored in the same place as the variable address? In my puny mind it seems that if there is this other index, then we are talking about something more like a heap? I'm clearly very confused, and I'm just hoping there is a simple answer to my simplistic question. Thanks for read