Questions & Answers about Umarte siya.
It is built from the root arte plus the infix -um-:
arte → umarte
The -um- is inserted after the first consonant of the root. This is a very common Filipino verb pattern.
In many Filipino verbs, -um- marks an actor-focus verb. That means the sentence is grammatically centered on the doer of the action.
In Umarte siya, the doer is siya. So the structure is focused on he/she as the one doing the acting.