Open any Google app or go to https://accounts.google.com
Click your profile picture in the top-right
Select Sign out
If you see multiple accounts, click the profile picture again and choose the account you want to sign out
Click Sign out for that account
To sign out from all Google sessions, go to https://accounts.google.com/Logout
On Android: Open Settings → Accounts → Google → select the account → Remove account (or Remove from this device)
On iPhone/iPad: Open Settings → Mail → Accounts → select the Google account → Delete Account
To sign out of a browser session: open the browser menu → Settings → Privacy/Security → Clear browsing data (cookies and site data) and refresh the page