EHAS FULLY VACCINATED DEFINITION

I find the "RULES" confusing.
Is a Booster shot required to sidestep test Y/N?

Yes. The red chart explains it best. The difference being fully vaccinated without the booster allows you to take an antigen test if you want. Non-fully vaccinated must take a rt-PCR test.

Yes Currently you need Booster within 9 months to not require test. That could change anytime so it's best to keep checking.

Thanks
Traveling now sure is a lot of work!

As the others have said, read the red chart on the official rules site. Link That is it in a nutshell. Pretty clear.