I have read answers about roots in sewer lines, and adding various chemicals that can help kill the roots.

What about the field lines in a septic system,

If chemicals are added into the line at the house, they will only "dump" into the septic. Will adding enough of this ever get into the actual field lines, are there other ideas how to do that.

You are correct Chuck, Good question!

Dumping root killer down your toilet only takes csre of the roots in the sewer line from the tank to the house.
For roots in the filter bed/drain field you must locate the exit point of the septic tank. There you will find a manifold that directs the liquid output to the different branches. That's where you have to insert the root killer.
Regards, Tom