If you are sending an Amended Return with Form 8606, the address to send the 1040X should be in your Filing Instructions.
If you are sending stand-alone 8606's you can send them to the same address you would send Form 1040.
On the actual PDF of Form 1040X, I don't see the checkboxes you are referring to. That may have been a TurboTax thing, that was changed in 2019.
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