"Bricking" is a Little bit wrong Word, because this refers to old theft locks on older phones, that really did make the phone unuseable, permanently. There was no recourse of activating it, not even for the manufacturer. Then they made so such locks could be reenabled by the manufacturer, but only after being the original owner and showing a sales receipt, or a new owner and showing a sales receipt and a handwritten receipt.
Still with lots of misuse, they made so locking requires a valid police report, and further from this, this was what became IMEI locks today.
However, on newer phones, IMEI locks are ineffective today, since the phones can be used without GSM coverage or SIM card, by using wifi in a closed enviroment. Thus the phone could be sold by the criminal as a "wifi-only phone" and still earn a lot of Money on the stolen Equipment.
There is a legilisation passed in the US that mandates such a feature making it possible to disable a phone, but the law also says the feature MUST be reversible for whoever did set the lock.
The reason the feature must be reversible, is that the feature (activation lock) should also be possible to be used by the police, to disable all phones in a specific area, or temporarly disable a suspect's phone during investigation.
The feature must also survive a bootloader replacement, so normally, the activation lock feature would also disable firmware updating, eg, you have to disable the feature to be able to update firmware. iOS does this automatically in the background (while "verifying firmware", and then reenables it on next boot if it was enabled).
For Android activation lock, I don't know exactly how the features handles firmware updates, but there is surely something similiar there. Thats why you must put in your Samsung account password to update the phone if you have activation lock enabled on a Samsung phone. On nexus this is handled automatically like the iOS. On Windows phone, the firmware update process only accepts signed firmware, which will require entering the username/password of the activation lock if you reset the phone, you cannot insert firmware that skips this check.
And now to your questions.
The activation lock on iOS, Windows Phone and Android, is designed to be cumbersome to misuse. This because the feature is linked to the account used to set up the phone, same account that is used to download apps. This means that the phone will be unuseable to a new owner unless he immediately switch accounts, which ensure the old owner CANNOT disable the phone after it has rightfully transferred owner.
This means theres no need to write a handwritten receipt of purchase of phone. (Old kill-switches required this to unlock the phone, the original sales receipt was not enough because that beared the name of the original owner, thats why Carriers made the rules stricter and said a valid police report must be presented to disable a phone, to prevent people who sold the phone from misusing the kill-switch)
This means that if you buy the phone in-person, you run the setup wizard while still on Place. If the activation lock is enabled, it will then ask for the old username/password, and you will have to have the old owner to enter them. Then the kill switch will be transferred into your position instead.
If you are purchasing the phone via the internet on Ebay, you can instead use this service to check the status:
https://www.icloud.com/activationlock/ (iOS) or https://account.microsoft.com/resetprotection (WP)
It will allow you to check the activation lock status Before purchasing the phone.
If the website show a clear status (no activation lock present), save this as evidence, because if the seller does activate the lock just prior to shipping the phone, you can do a not-as-described claim on the Product, presenting evidence that the phone was not in a stolen state.
A clear activation lock will allow you to factory reset the phone if the owner's account is entered in the phone. A set activation lock will require the old account details when running the setup wizard from a freshly reset phone.
For android and samsung, theres no way to remotely check the activation lock, so don't buy these phones over the internet.
Removing a android activation lock on the Nexus variant, delete all google accounts in the account manager. Then do a hardware reset on the phone.
Removing a samsung activation lock is easy as going into the security menu and disabling the activation lock, you will be asked for samsung password. After that, the phone can be hardware reset to allow the phone to be freshly set up again.
Both these must be done in-person with the old owner present.
So really, those kill switches will work two-fold:
1: It locks the current software. This requires a remote command to unlock on most models, knowing a password or PIN is not enough.
2: It prevents reactivation of NEW software on the phone, if you don't have the username/password for the account that the activation lock was set up with.