For the most part, ARMv8 immediates are 12 bits for [base + displacement] addressing. However, most of the modes also allow you to use natural scaling of the displacement (or index in [base + index] modes). So you can reach the 4096th byte, or the 4096th pointer in a struct (and other sizes in between).

That 128 MB limit is interesting. Its the limit for direct pc-relative calls in ARMv8 without resorting to adrp.