Your not giving us mush context to work with but if you are looking for an sql query here is one that should work.

SELECT json_agg(to_json(f)) AS fruits FROM Fruit f;

SELECT json_agg(to_json(v)) AS vegetables FROM Vegetables v;

SELECT json_agg(to_json(p)) AS pasta FROM Pasta p;

Yes I'm sorry, this is the schema:

model Category {
  id          Int       @id @default(autoincrement())
  title       String
  description String
  color       String

  user_id     String
  fruits Fruit[]
  vegetables Vegetable[]
  pastas Pasta[]
}

model Fruit {
  id                   Int       @id @default(autoincrement())
  title                String       
  created_at           DateTime

  user_id              String
  category_id          Int
  category             Category? @relation(fields: [category_id], references: [id])
}

model Vegetable {
  id          Int       @id @default(autoincrement())
  title       String       
  created_at  DateTime

  user_id     String
  category_id Int
  category    Category? @relation(fields: [category_id], references: [id])
}

model Pasta {
   id          Int       @id @default(autoincrement())
   title       String      
   created_at DateTime

   user_id     String
   category_id Int
   category    Category? @relation(fields: [category_id], references: [id])
}

User is from Clerk, so I just have the reference to the string.
And I would like to put all this objects in only 1 JSON, so just 1 query, and this in return:
{
"fruit: [ {id: 1, title: "apple", created_at: "12:02:24 12-20-04" }, {} ],
"vegetables: [ {}, {} ],
"pasta: [ {}, {} ],
}