how get user_id from given acces_token to rails API ( I use google omniAuth)

Question

I hope you are all well.

I'm creating rails API and I include Google OmniAuth (it's only way to log in to my app). I successfully generate access_token, user can log in with google but.. after that how should I define user_id in my BE ? I know it cannot be the same as I have in my token info. Before I implemented google omniAuth, I used SecureRandom.hex