I keep a spreadsheet of all the Medical, Dental, Vision and Prescription expenses. There is a column to show what Medicare was billed, what Medicare approved, the amount Medicare paid, what the Medigap paid and the date paid. I also show if we paid something toward our deductible or out of pocket and the date paid and payment method.
I'm sure this is far more detailed than would be required but I like details.
At year end I print this out for my files and I also save the year end credit card recap along with all the receipts for things we paid.
Before Medicare this was a much bigger project as we paid for a lot more things because of ACA high deductibles. With Medicare we pay for very few things.
We've had our HSAs since 2014 and only reimbursed once or twice in lump sums, never for every little thing throughout the year. We don't need the reimbursements so I keep our HSA as if they were extra Roth accounts.
I think about this in terms of if the IRS ever questioned an HSA reimbursement, what info would they need - What was the expense for, how much were we billed, how much did we pay, how was it paid.
I could make this much simpler, but like I said, I like details