Peel and devein the shrimp. Save the heads and shells for the sauce.
Soak the noodles in tap water for 10 minutes. Drain, then add the noodles to a pot of boiling water. Cook for 10 minutes, then turn off the heat and let them sit in the hot water for 15-30 minutes, or until tender. Drain and set aside.
Cook the shrimp heads and shells in a little oil until lightly browned, pressing on the heads. Add 8 cups water, salt, pepper, and a bay leaf. Bring to a boil, then simmer for 5 minutes. Strain the stock and discard the shells.
Boil salted water. Add shrimp and cook for about 1 minute, until pink and opaque. Remove and set aside.
Add squid rings and cook for 30 seconds to 1 minute, just until opaque. Drain and set aside.
Boil the eggs for 10 minutes. Transfer to cold water, then peel and slice.
Brown the ground pork in a wok or large pan, breaking it up as it cooks. Add oil if needed. Season with salt.
Add a little oil. then sauté the garlic and onion until softened. Add the tinapa flakes and cook until lightly browned.
Add the annatto powder, calamansi or lemon juice, and shrimp stock. Season with fish sauce and black pepper. Taste and adjust. Bring to a boil.
Mix the cornstarch with ½ cup cold water until smooth. Pour into the sauce while stirring, then simmer until very thick.
Add the noodles, napa cabbage, and crushed chicharon. Toss until evenly coated. Add a little hot water if needed.
Top with the shrimp, squid, sliced eggs, more chicharon, and scallions. Serve with calamansi or lemon on the side.