Glossary
Link register (x30 / lr)
The AArch64 register that holds the return address; bl writes it and ret branches to it, so the return address is not on the stack by default.
The link register (x30, also written lr) holds the return address on AArch64. The branch-with-link instruction bl sets x30 to the address after itself, and ret branches to whatever x30 contains. A leaf function that calls nothing never disturbs x30, so it has no return address on the stack to overwrite.
A non-leaf function must preserve x30 across the calls it makes, so its prologue saves x29 (frame pointer) and x30 to the stack, typically with stp x29, x30, [sp, #-N]!. That saved copy is what a stack overflow overwrites to hijack control, and what pointer authentication signs to prevent forgery. See the link register and ret2win.