Abu,
U have to start probing for the lines and dig up portions to find the line patterns. If the heads patterns don't help then start at the system valves and work outward. U should create a plan of your system. U say hose to the plants. Are these garden hose types or black flexible hose for with thin walls? Or are they hard pvc pipes? Do u know where the auto-valves and manifold are by the water source? Is the water source from your outdoor water faucet or underground with a valve shutoff? U also need to know your water pressure to the yard and flow rate. If u need to know how to find that out we can help on this forum. GL aloha

Is it a drip irrigation system or sprinklers? Either way most clogs happen at the water emitters not in the lines. Remove the emitter/sprinkler and see if water comes out. if you are getting water clean or replace the emitter/sprinkler.

Balancing a watering system can be difficult. You would have to match the flow of the new system you are putting in with the flow of the current system. I would recommend just putting in a new circuit with its own solenoid valve control instead of trying to patch into the e3xisting system.