I never heard of zucchini relish, but I do love having relish on my hot dogs.
He does a great job explaining.
He has many canning tutorials on his channels that are useful too.

Image by Unemployed Redneck Hillbilly Creations via YouTube
Ingredients:
Directions:
First, wash your zucchinis and trim the ends.
Chop the zucchinis into small and fine pieces.
Dump all of the veggies in a large bowl and mix them all together.

Sprinkle some pickling salt and cover with cold water to sit for 2 hours.
After 2 hours, drain the veggies and rinse with cold water.
Next, add the veggies to the water-vinegar mixture and simmer for 10 minutes.

Stir occasionally so nothing burns on the bottom of the pot.
ensure the jars are clean and preheated before filling them up.
Take a funnel and fill up each jar with 1/4 headspace.
Wipe off the sealed surface before sealing.
Overall, canning vegetables are popular for those that have a lot of veggies stocked up in your refrigerator.
I personally never had zucchini relish, but Im sure it tastes just as good.