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Base on my understanding, isn't someone able to know your total holdings when they check one of your address using walletexplorer.com?
no.
Walletexplorer will only know which addresses belong to the same wallet if you create a transaction and use several inputs funding different addresses.
If you have 10 addresses that were funded in the past but they were freshly generated and no unspent outputs funding those addresses were ever spent, let's call them 'address_1'...'address_10', walletexplorer will not know they belong to the same wallet.
But, if you ever create a transaction and use unspent_output_1 funding address_1 AND unspent_output_2 funding address_2, those explorers will know address_1 and address_2 belong to the same wallet. They'll have no idear address_3...address_10 also belong to this wallet.
Things do get a bit more complicated if you look at change addresses, those have to be labelled to... Otherwise you'll spend change from a transaction with inputs to a different address, and they'll know those addresses belong together to.
Walletexplorer also searches publicly available owner info, that's how they know wallets from big exchanges
Case in point: this is an address of mine that's publicly known: bc1q50udcgfdyqanp56m9dkcqkxy5fayjc74vw9px7
Walletexplorer was able to deduct this wallet contains at least 5 addresses:
https://www.walletexplorer.com/wallet/60911730d7ab735d/addressesIn reality, this is a wallet i've been using since 2019, currently i'm at address 125, and change address 72. So my wallet contains 197 addresses that were, at least at one point in time, funded. Walletexplorer has been able to group those addresses together whenever i created transactions using inputs funding more than one address. So they created groups of ~1-~5 addresses, whilst having no clue my wallet actually derived ~200 addresses (so far)