Disk Utility Erase - What does it do?

I will be selling MacBook Air soon. If I run the Erase function of Disk Utilities will this delete all my data? (KeyChain files, Applications, Network Settings, etc.)

Will the Erase function leave the computer is a condition where the new owner can boot the computer as if it were a new purchase and create accounts, passwords, etc.?

Am I correct in assuming the Erase function will not downgrade the OS from 10.13.3 to actual OS when I purchased the MBA?