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But what's the mechanic behind ? I mean I understand I can create my own btc wallet, send to other ppl btc wallet using their address, but how do those shops generate address like this and can recover the btc behind ?
Do they have internal mechanics that create a new wallet they own once I registred ?
So they must own tons of wallets (1 per customer) running arround ?
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If you ask about the wallet address in the site, it just the same process most wallet are using especially if its an HD one. Sometimes online shops do use payment processor such as btcpay server. The idea is, in every user registered the site generates a unique wallet address using the payment processor's API/program and it will be saved to their database for easy identification of every user. So yes, it may generate millions of wallet address depends on the number of the website's users.