When ram says full what happen? Phone not lagging.

[AC Question]

What happens when ram reads full. Phone runs good. No lagging.

 

[ManiacJoe]

Android likes to have all the memory in use, unlike some other operating systems.
If the memory is full and more is needed, Android will pick a "not recently used" app to swap out and give that memory to the app that needs it.
Some apps, like many games, can be sensitive to the time it takes to do that, if the app wants a HUGE amount of new memory resulting in the need to swap out multiple apps.

Running out of internal storage (disk) space is a very different thing, and a very bad thing.