look at form 8606. you lump all IRA's, SEPs and Simple IRAs together. similarly, your tax basis would be the sum of the tax basis in each of these accounts
this can be an oversimplification but it shows the general principals
assume you have tax basis in IRAs of $10K. at year-end, you have other similar retirement accounts with a fair market value of $100K (assume zero tax basis). during the year you take a $20K distribution. the account from which it comes doesn't matter. the non-taxable portion is the distribution amount of $20K distribution divided by the $100K FMV times your basis of $10K meaning $2k isn't taxable and $8K is. the $2K of basis used reduces your basis for next year to $8K
it's your responsibility to keep track of tax basis and best to file form 8606 each year to show your tax basis even if no distributions are taken.
