I would suggest you looking Sycra or Proko. Sycra is more of semi-realistic and explains a lot about how to shade and use your colors correctly. Proko explains more about realistic drawing, with examples and step by step guide, will post again when I think of some artists again.
There are many ways to understand the basics of realistic art, Youtube being one of them. But do you mean on a software or drawing (irl) because they both have their ups and downs like, software you can customise settings to your liking and to fit the art piece you plan to make, irl you have TOTAL control over what you draw - this can be more helpful if you're unfamiliar with software and can't use it to the full extent but have a good idea of what it is you're trying to create.
*Pulls more tail then a special needs child at the petting zoo*
it depends what you want to draw realistically. different subjects require different techniques
The past makes you wanna die out of regret, and future makes you depressed out of anxiety. So by elimination, the present is likely the happiest time.