b.Ex1:Understood. (not “understand” alone. Is it because of no subject and therefore switches to passive even if meant in present tense?) As in "It is understood" (subject "It") and portion of the passive ("is) is simply assumed. But you will hear, on occasion, "Understand" as a response to some comment.
Ex2:I understand. (seems this is
theanother alternative but with subject taking the action thus “understand” is in active.)
The context is response situation at least.
Seeking explanation/confirmation for example 1.
Student or Learner