Create macOS High Sierra and newer bootable recovery partition on a separate standalone disk There are manuals on cloning the recovery partitions to a separate disk for any macOS or OS X version prior to High Sierra, but I can google nothing about creating a standalone recovery disk for High Sierra and higher.
Is it possible for APFS based macOSs? Are there any manuals?
What you suggest is no longer possible with Big Sur. When booting to Recovery, you are required to enter a password of an existing account. AFAIK, these passwords are not stored in the Recovery volume. This would make it impossible to boot from a standalone Recovery disk.
Before Big Sur, there is no reason the an AFPS Recovery volume could not be placed on a standalone disk. However, getting the firmware to boot Recovery from such an arrangement might require a third party boot manager such as rEFInd. I an unaware of any manual which would explain how to do this. On the other hand, I can not imagine a reason one would want to do what you are proposing.