According to the Bluetooth Core Specification Version 5.2 | Vol. 1, Part A, Chapter 5.1 pairing is defined as
the process for creating one or more shared secret keys
The keys demselfes are not shared but established using a procedure called Diffie-Hellman key exchange. This means that pairing is only necessary if a connection is encrypted. If you connect to a device that requires security measures on one of its characteristics Android will prompt the user of your app with a pairing request automatically.
Another term you might have heard is bonding, which is defined as:
the act of storing the keys created during pairing for use in subsequent connections in order to form a trusted device pair
Bonding allows a reconnection of your devices without another pairing process since the keys have been established before.
When using Bluetooth Low Energy (BLE) pairing a manually Device within the OS is not needed. You can always scan for a device that advertises its presence. As I said before: Pairing is handled by the OS automatically if needed.
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