During a low-fee environment, spend the smaller UTXOs for privacy preservation (recall that people can see the size of your UTXO when you spend on-chain, so the smaller the better from a privacy standpoint).

During a high-fee environment, spend a single, larger UTXO to minimize fees (fewer UTXOs = lower vBytes = lower txn fees).

In all cases, only spend UTXOs that have already been mixed via collaborative transactions (aka coinjoins) for forward looking privacy preservation.

Also in cases, minimize UTXO “dust” by consolidating UTXOs from time-to-time so none of your UTXOs are less than 300K sats, or even better, 1M sats.

Granted, these are rules of thumb only.

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But how do you choose to spend a specific utxo in a wallet?

Choose the UTXO that is the closest size to the amount you intend to spend, though it must be slightly larger to account for the transaction fee.

You can select a specific UTXO using any wallet that has that feature (Sparrow Wallet, Blue Wallet, etc)

Think of it like $20 bills, $10 bills, $1 bills.

If you’re buying something that’s $9, use a $10 bill.

One difference, however, is that in fiat world, you get charged based on the dollar value, whereas in Bitcoin world, you get charged based on the vByte file size. So you want to use the minimum number of “bills” or UTXOs that will cover the transaction, regardless of the dollar amount, to minimize fees.

No, like litterally "how" I don't know of anyway to say " use this utxo for this transaction" instead of the wallet automatically choosing

You need to use a wallet that has that feature, as I said.

Sparrow Wallet is my favorite for this.

Didn't see your other reply. Do any mobile wallets have this feature?