You're right about the affiliate model. But most NPR stations in all but the largest markets predominantly run nationally syndicated content. Creating and syndicating that content is the NPR mothership's largest cost center, averaging 58% of operating expenses FY18-FY22.

Corporate sponsorships are, at an average 39% during the same period, the largest source of funding. Contributions (what we'd call donations) are 12%.

Put another way: Local donors may (or may not -- I don't know) form the backbone of funding individual stations. But corporate sponsorships fund most of the content that appears on most of the stations, and also funds its distribution.