Automation testing based on API services level instead of web UI

I start to test new project, and the developer send API services to me to test it because frontend(UI) implementation is not ready. I have to write Automation scripts for features after stabilizing them

• Is it possible to automate API services functionalities, and how plz?
• or I have to wait untill frontend is ready so I can locate elements and automate through UI.
• Can you please explain to me difference between Automate through WEB UI vs through Automate API