At Google, the vast majority of new college hires would begin as Software Engineers at Tech Level 3 (L3), where Tech Level 5 is "Senior Software Engineer." Level 6 is "Staff."
The terminal Level concept was one where all Software Engineers are expected to perform in an upward trajectory toward the standards of the next Tech Level, until reaching Tech Level 5. I.e. Software Engineers are expected to be promotable up to Level 5, and after reaching L5, there is no longer an "up or out" expectation. L5 SWEs are not expected to become Staff Engineers.
Today, the Terminal Level for Google SWEs is now Level 4.
The median Google SWE historically was around L4, and your average Google SWE would be a seasoned L4, potentially ready for a promotion to L5 in the next few promotion cycles.
The idea of a "principal" Software engineer is Level equivalent to a Director (Level 8) of which despite there being many at Google/Alphabet, as a percentage of the total population it is quite small.