In the US, the IRS treats bitcoin as property. When you pay for something with sats, you've sold those sats for the value of what you bought. If your bitcoin went up in value since you bought it, that difference is a capital gain, even on a $6 coffee.
gain = value of purchase − (sats spent ÷ 100,000,000 × your cost per BTC)
Bitcoin you held for more than a year produces a long-term gain, taxed at lower rates. A year or less is short-term, taxed like regular income.
General information for US taxpayers, not tax advice. Proposals for a small-purchase exemption have been discussed in Congress; check current rules or ask a CPA before you file.