Android one device update/upgrade?

[AC Question]

I couldn't find any exact information anywhere.So,I am asking here. How long is an android one device updated or upgraded? As far as I know,an android one device gets upgraded for 2 years.After 2 years It doesn't get any major update.How about security patches after 2 years?Does an android one device get security patches after 2 years of its introduction?How long does an android device get security patches?