What is the process of removing introns and joining exons in a defined order during transcription called?

$DNA$ transcription unit is defined primarily by three regions in $DNA$ with respect to upstream and downstream ends:

In eukaryotes,the $RNA$ polymerase that synthesises $tRNA$ is $RNA$ polymerase . . . . . . and is also responsible for formation of . . . . . . $rRNA$.

Define the following terms: $Cistron$ and $Terminator$.

Define/Explain: $A$ promoter and the structural gene.

In which of the following places is messenger $RNA$ $(mRNA)$ formed in a living cell?