Struct member access?

[pseudo grail]

in the first function you're reading the bytes that come after the header field; in the second function you're reading the bytes that the tables field points to.
both functions are quite hard to understand - can you explain what you're trying to do, and where do the data come from?

findTable
+--------+--------+
| header | tables |
+--------+--------+
         ~~~~~~~~~~~~~~~ reading this

findTable1
+--------+--------+
| header | tables |
+--------+--------+
             |
             |
             |      +-------------+
             +----> | ... ... ... |
                    +-------------+
                    ~~~~~~~~~~~~~~~ reading this

if findTable (no 1) is the working version - you have done your type-modelling wrong.

"type modelling" as in, writing Zig types that represent the meaning of some sequence of bytes, according to some format

the tables field in the first code snippet is wrong - it represents a pointer to some remote data; when you actually want to represent a sequence of inline elements, in the classic header-body format

Zig does not have flexible array members, I suggest you completely remove the tables field and instead make a utility method:

const LengthPrefixedString = extern struct {
    len: u64,
    body: void,

    fn getSlice(self: *const LengthPrefixedString) []const u8 {
        const ptr: [*]const u8 = @ptrCast(&self.body);
        return ptr[0..self.len];
    }
};

memory representation:
+--------+----+----+----+-/ /-+----+
| length | u8 | u8 | u8 | \ \ | u8 |
+--------+----+----+----+-/ /-+----+
         |-------------------------| length given by the len field