Make the pickled red onions. Whisk the apple cider vinegar, sugar and salt with 1 cup water in a small bowl until sugar and salt dissolve. Place onion in a jar; pour vinegar mixture over. Let sit at room temperature for 1 hour.
Place the pork in a large dutch oven with the orange juice, lime juice, salsa and salt. Add enough water to barely cover the meat. Bring to a boil. Once boiling, reduce heat to medium low and simmer with the lid on for 2 hours, at which point the pork should be falling apart to the touch.
Turn the heat up to medium high, and continue to cook the pork for 45 minutes, uncovered, stirring every once in a while, until all the liquid has evaporated and the pork fat as been rendered.
Parts of the pork should be caramelized and golden brown. Remove the dutch oven from the stove and season with salt and pepper as needed. Serve as needed or let cool and store in an airtight container for 4 days in the fridge
To assemble the tacos, warm or char a small tortilla and place a few pieces of carnitas on top. Add a dollop of guacamole, a few pickled red onions, chopped onions, jalapenos and a sprinkle of cilantro. Serve immediately.