welcome to the conversation. In answer to your questions:

1. ) To dry basil leaves: harvest small branches in evening before dew falls, from plants that have not yet started to flower. Dry on screens in a warm dark place - not hotter than 100 degrees. Most dehydrators run too hot, but ovens with pilot lights ( or electric ovens with lightbulbs) work well as long as temps are monitored; you may have to keep the door open a crack. An ordinary room thermometer will give you a more accurate reading than a culinary one, btw. oh, very important: dry JUST until breakable, then pack in airtight jars. Overdrying dilutes flavor and plastic isn’t airtight..

2.) Basil pants are tender annuals that will live until they set seed or until frost, whichever comes sooner. But they won’t stay useful for more than about 6 weeks unless you are fanatical about pinching them back and even then they’ll probably start getting tough and strong after a couple of months. Basil isn’t usually on the list of vegetables that should be started from seed several times through the summer, but it should be. Germination and growth are quick in hot weather as long as the plants have water.