smartphones are necessary to pay via UPI
smartphones--bank accounts--aadhaar card
in case of some low income workers, smartphones are not universal
in case of some migrant workers (including those that come from neighboring countries in search of jobs) .. aadhaar or bank account may not be immediately available
while digital payments are definitely becoming common cash is still accepted
in some cases autorikshaw drivers either avoided UPI (because something went wrong previously and their account went into negative balance ..so any payments they accept there will be swallowed by the app) ..or sometimes ask for cash as preferred mode because for them it is easier to spend immediately (sometimes even for sentimental reasons like first ride of the day)