Can I "duplicate" a wallet using a backup phrase (12 words)? My understanding is that a wallet isn't a wallet at all*, it doesn't hold my coins, but its a key store; my coins are of course &quot;stored&quot; as records of the events on the blockchain.
Imagine I have bitcoins with Cloud Wallet Company (A), but I then setup Wallet Company (B)'s app. And let's assume they both use the same crypto algos to compute the seed from the 12-word recovery key, as per this answer.
Is the &quot;recovery phrase&quot; generated by a wallet app platform independent?
If I login to (B) and choose to import/recover my wallet and punch in the 12 words I got from (A), would I effectively now have two wallets? i.e. both (A) and (B) are setup with the same wallet.
Where is my balance stored? After entering my 12-words into (B) would their servers have to &quot;run a query&quot; on the blockchain to aggregate all the transactions and arrive at my balance??
And if I send from (A) to a friend's wallet (Z), would my reduced balance also show on (B)? Or would (B) cache my balance to avoid expensive query/compute from the blockchain, and so it would become out of sync?
Thanks!!
*Maybe I'm wrong but I think the wallet analogy only confuses noobs, since it sets up a mental model that's the opposite of the whole point of a blockchain.
Can I “duplicate” a wallet using a backup phrase (12 words)?

Yes

Where is my balance stored?

Both wallets will scan the relevant parts of the public  transaction-journal (&quot;blockchain&quot;) to add up the amount of money controlled by the keys derived from your backup phrase - this total is your balance.
Wallets are very likely to store this total locally, as a convenience, so they don't have to re-scan the blockchain and add up the total all over again every time the wallet is opened. Exactly where and how this number is stored is likely to vary depending on the software used. The storage of this total is of no real consequence.

if I send from (A) to a friend's wallet (Z), would my reduced balance also show on (B)?

Yes, both wallets will learn about new confirmed transactions that affect your balance.
Wallet A might show the transaction amount as pending until it receives a confirmation, a confirmation is just a new block appearing in the public transaction journal that either contains the transaction in question or which builds on top of such a block. Wallet B learns about the confirmation in the same way and will similarly notice that the block contains a transaction that affects an amount associated with an address derived from your backup phrase. The mechanism is the same for both wallets, Wallet A has no special privileges.
To elaborate a little:
As Murch noted in a comment, wallet A might have some additional information that doesnt get transmitted to wallet B. This is information which isn't transmitted to the rest of the Bitcoin network and which has no effect on validation of a transaction or on the mining process by which payments are considered to be confirmed. One example is if you choose to add a memo &quot;payment for new trinket&quot; to the transaction, this memo is not transmitted to anyone else and so is not known to Wallet B (or anyone else). It is also lost if you destroy wallet A and re-create it (e.g. by deleting an app and associated data and then reinstalling the app and using the old backup phrase). If your wallet A allows you to organise transactions into accounts, wallet B wouldn't automatically know about those accounts. Similarly, if wallet A is a Lightning-capable wallet, Wallet B wouldn't normally know about the lightning transactions that Wallet A made, only about the final on-chain settlements made e.g. when a lightning channel is closed.
However Wallet B should still normally show that same total as Wallet A for total amount of Bitcoins that the basic Bitcoin network and it's public replicated transaction-journal (&quot;blockchain&quot;) consider to be controlled by private keys which are in your wallets A and B.