How do banks (and other websites) create one-time passcodes? (OTPs)

It used to be that banks (and other sites) would text or email 4-digit codes (8374).

Now almost all have gone to 6-digit codes (287535), and a few have gone to alphanumeric codes of various lengths (like this: Yn8wrDeJ)

Are these truly random? Are the numeric codes some sort of algorithm that takes this exact millisecond and uses the last 6 digits? What about the alphanumeric ones – how are they produced?

(observation: recently almost all of the 6-digit codes I’ve received have had at least one number repeated (884709 or 547045) which I think makes them easier to remember.)

Does anyone know how they are produced?