how do I unlock the passcode from a lost phone that I have found?

If the iPhone is showing "iPhone disabled - Try again in..." give your best try until you see the message "iPhone disable - connect to iTunes." This way you don't lose the data. If that doesn't work, erasing is the only option.

Use same Find My to erase your iPhone if it's connected to wifi or cellular network.

https://support.apple.com/guide/icloud/mmfc0ef36f/icloud#:~:text=In%20Find%20My%20iPhone%20on%20iCloud.com%2C%20click%20All%20Devices,Click%20Erase%20%5Bdevice%5D.

Or restore the iPhone with a computer.

https://support.apple.com/HT204306

GCarter1803 Said"

"I lost my own phone and found it on Findmyphone. The airport people that found my phone were nice enough to mail it to me , but I can’t remember my own passcode because I used the thumbprint for so long..... no need to call the authorities"

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I'm not saying to a call them, as if it is an emergency. Hand it over, in person -- your call.

GCarter1803 Said:

"how do I unlock the passcode from a lost phone that I have found?"

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You don't. Just be responsible about it, and hand it in to local authorities.