Question: How do I come to a main plot while balancing sub- plots? In this case: a murder mystery?Answer:
Well, if it's a murder mystery, then I expect the main plot will be about someone being killed and someone else's efforts to find out who did it. It will likely also include the killer's attempts not to be caught, and the reasons why that person became a killer.
Usually the murder is the inciting incident. The investigation is the complication phase, the deduction is the climax, and the outcome is the final tying up of loose ends.
The other subplots are usually there to distract the detective and/or the reader with red herrings.
How you weave back and forth among these plotlines is up to you.
One time tested technique for working this out, if you are a plotter, is to write out each plotline on index cards - one card for each event in the plotline. You can use a different colour of cards for each plotline.
Lay out each plot as a row of cards going from the beginning to end. Then combine all the cards into one large stack, in the order that the events will appear in your novel.
You can include backstories as part of the plotlines too, just to work out the chronology of what happened before the novel begins. However, when arranging the cards, you may need to put backstory events in as "revelations" later in the book.