If, for example, you are at checking out at Home Depot and they ask you “would you like to round up your total to the nearest dollar to benefit x__________ charity” DONT do it. They are taking that money and donating it in their name and using that donation to be tax exempt.
If you feel so inclined, donate it personally and help hold these mega corporations accountable for what little taxes they do have to pay.
sadly a lot of these companies also donate to legit charities, so a lot of people will just hit "donate" because they know the org is a good one and deserves the money but the opportunity cost of doing so is literally tax evasion