Simple custom JSON decoding in Swift for specific keys. Hot tip is to nest the CodingKeys similarly to how the JSON is nested.

CustomDecoding.swift

struct Data: Decodable {
    var age: Int
    var address: Address
    
    struct Address: Decodable {
        var street: String
        var city: String
    }
    
    private enum CodingKeys: String, CodingKey {
        case name, data
        
        enum DataKeys: String, CodingKey {
            case age, address
            
            enum AddressKeys: String, CodingKey {
                case street, city
            }
        }
    }
    
    // Implement a custom initializer that takes a Decoder object
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        
        // Data
        let dataContainer = try container.nestedContainer(keyedBy: CodingKeys.DataKeys.self, forKey: .data)
        
        // Age
        self.age = try dataContainer.decode(Int.self, forKey: .age)
        
        // Address
        let addressContainer = try dataContainer.nestedContainer(keyedBy: CodingKeys.DataKeys.AddressKeys.self, forKey: .address)
        let street = try addressContainer.decode(String.self, forKey: .street)
        let city = try addressContainer.decode(String.self, forKey: .city)
        self.address = Address(street: street, city: city)
        
    }
}

let json = """
{
    "name": "John",
    "data": {
        "age": 30,
        "address": {
            "street": "123 Main St",
            "city": "Anytown"
        }
    }
}
""".data(using: .utf8)!