Reserves are created when the central bank buys assets and destroyed when it lets them run off. The current framework aims for ample reserves, meaning enough that banks do not have to bid aggressively for them and iorb can steer effr directly.
The hard part is that nobody knows in advance where ample ends and scarce begins. balance-sheet-runoff proceeds until funding markets show stress, as they did in September 2019.
Example: reserves fall from $3.3 trillion to $2.9 trillion during QT. GC repo starts printing above iorb at month-ends, and the Fed slows the pace of runoff in response.
Related: iorb, balance-sheet-runoff, overnight-reverse-repo-facility, standing-repo-facility, effr