How to create the dynamic struct except using interface and Using spread operator

I am trying to create the new API with Go and Fiber framework. I am using MongoDB.
Please refer to my below structure

type Product struct {
	Name           string              `json:"name"`
	Code           string              `json:"code"`
	Description    string              `json:"description"`
	Brand          string              `json:"brand"`
	Variants       []map[string]string `json:"variants"`
	Categories     []int               `json:"categories"`
}

The Variants field will have another set array of objects. But the fields will be dynamic. So I can't define the structure. Now this one I solved with []map[string][string].

My task here I have to form the CSV file with the below format.

[{
	Name: "Rock",
	Code: "RRR"
	Description: "This is rock"
	Brand: "R"
    ...variants
},
{
	Name: "Rock",
	Code: "RRR"
	Description: "This is rock"
	Brand: "R"
    ...variants
},
{
	Name: "Rambo",
	Code: "RAM"
	Description: "This is Rambo"
	Brand: "RA"
    ...variants
},
{
	Name: "Rambo",
	Code: "RAM"
	Description: "This is Rambo"
	Brand: "RA"
    ...variants
}]

Only variants will change for every row. Others will be remaining same
I formed the other fields except for the Variants. I can't create the struct because the data will be dynamic.

I have a lot of confusion

1. How to create the struct for dynamic fields
2. How to spread the Variants at the root level

I am from javascript. So, we used the spread operator. Here I am confused about how to do that in Golang.

Help me out, guys

@mkopriva is right
>However you can use reflection to dynamically construct a new struct if you like

follow is a example:

package main
import (
"encoding/json"
"fmt"
"reflect"
)
type DynamicMap map[string]interface{}
type Product struct {
Name        string            `json:"name"`
Code        string            `json:"code"`
Description string            `json:"description"`
Brand       string            `json:"brand"`
Variants    map[string]string `json:"variants"`
Categories  []int             `json:"categories"`
}
func (j Product) MarshalJSON() ([]byte, error) {
m := DynamicMap{}
t := reflect.TypeOf(j)
v := reflect.ValueOf(j)
for i := 0; i < t.NumField(); i++ {
if t.Field(i).Tag.Get("json") == "variants" {
dyn := v.Field(i).Interface().(map[string]string)
for key, val := range dyn {
m[key] = val
}
} else if t.Field(i).Type.Kind() == reflect.String {
m[t.Field(i).Tag.Get("json")] = v.Field(i).String()
} else {
m[t.Field(i).Tag.Get("json")] = v.Field(i)
}
}
return json.Marshal(m)
}
func (j Product) UnmarshalJSON(data []byte) error {
fmt.Println("Unmarshal...")
return json.Unmarshal(data, &j)
}
func main() {
p := Product{Name: "aa", Variants: map[string]string{"a": "a", "b": "b"}}
marshal, err := json.Marshal(p)
if err != nil {
fmt.Printf("%v\n", err)
}
fmt.Printf("%s\n", marshal)
}

{"a":"a","b":"b","brand":"","categories":{},"code":"","description":"","name":"aa"}