Season & load: Toss pork cubes with salt, cumin, oregano, and pepper. Place in Instant Pot with garlic, onion, orange juice + spent halves, lime juice, and broth.
Pressure-cook: Seal and cook 35 min on High. Allow 10 min natural release, then quick-release.
Shred: Remove pork to a bowl and shred with two forks, discarding large fat pieces. Reserve ¼ cup cooking liquid.
Crisp, optional but authentic: Spread shredded pork on a rimmed sheet, drizzle with reserved liquid, and broil 5 min, stirring once, until edges caramelize.
Assemble: Spoon hot carnitas into lettuce cups. Top with diced onion, cilantro, and a squeeze of lime.