form_with でリレーションしているテーブルにもデータ登録したい

解決したい事

リレーションしているテーブルにも同時にデータ登録する方法（コードの書き方）が調べてもわからず困っております。
記事を見ても解釈できないため、わかりやすく解説している記事やサンプルコードがあれば紹介頂けたら幸いです。

accepts_nested_attributes_forを使わず、複数の子レコードを保存する

環境

Ruby 2.6.3
Rails 6.0.0.rc1

ruby
1#/views/users/_form.html.erb
2<%= form_with(model: user, local: true) do |form| %>
3  <% if user.errors.any? %>
4    <div id="error_explanation">
5      <h2><%= pluralize(user.errors.count, "error") %> prohibited this user from being saved:</h2>
6
7      <ul>
8        <% user.errors.full_messages.each do |message| %>
9          <li><%= message %></li>
10        <% end %>
11      </ul>
12    </div>
13  <% end %>
14
15  <div class="field">
16    <%= form.label :name %>
17    <%= form.text_field :name %>
18  </div>
19
20  <div class="field">
21    <%= form.label :address %>
22    <%= form.text_field :address %>
23  </div>
24
25  <div class="field">
26    <%= form.label :age %>
27    <%= form.number_field :age %>
28  </div>
29
30  <div class="actions">
31    <%= form.submit %>
32  </div>
33<% end %>

ruby
1#/controllers/users/users_controller.rb
2class UsersController < ApplicationController
3  before_action :set_user, only: [:show, :edit, :update, :destroy]
4
5  # GET /users
6  # GET /users.json
7  def index
8    @users = User.all
9  end
10
11  # GET /users/1
12  # GET /users/1.json
13  def show
14   # @user = @user.companies.build
15  end
16
17  # GET /users/new
18  def new
19    @user = User.new
20  end
21
22  # GET /users/1/edit
23  def edit
24  end
25
26  # POST /users
27  # POST /users.json
28  def create
29    @user = User.new(user_params)
30
31    respond_to do |format|
32      if @user.save
33        format.html { redirect_to @user, notice: 'User was successfully created.' }
34        format.json { render :show, status: :created, location: @user }
35      else
36        format.html { render :new }
37        format.json { render json: @user.errors, status: :unprocessable_entity }
38      end
39    end
40  end
41
42  # PATCH/PUT /users/1
43  # PATCH/PUT /users/1.json
44  def update
45    respond_to do |format|
46      if @user.update(user_params)
47        format.html { redirect_to @user, notice: 'User was successfully updated.' }
48        format.json { render :show, status: :ok, location: @user }
49      else
50        format.html { render :edit }
51        format.json { render json: @user.errors, status: :unprocessable_entity }
52      end
53    end
54  end
55
56  # DELETE /users/1
57  # DELETE /users/1.json
58  def destroy
59    @user.destroy
60    respond_to do |format|
61      format.html { redirect_to users_url, notice: 'User was successfully destroyed.' }
62      format.json { head :no_content }
63    end
64  end
65
66  private
67    # Use callbacks to share common setup or constraints between actions.
68    def set_user
69      @user = User.find(params[:id])
70    end
71
72    # Never trust parameters from the scary internet, only allow the white list through.
73    def user_params
74      params.require(:user).permit(:name, :address, :age)
75    end
76end

ruby
1# /models/user.rb
2class User < ApplicationRecord
3    has_many :companies
4end
5
6# /models/company.rb
7class Company < ApplicationRecord
8    belongs_to :users
9end

ruby
1# db/schema.rb
2ActiveRecord::Schema.define(version: 2019_05_17_051133) do
3
4  # These are extensions that must be enabled in order to support this database
5  enable_extension "plpgsql"
6
7  create_table "companies", force: :cascade do |t|
8    t.string "name"
9    t.datetime "created_at", precision: 6, null: false
10    t.datetime "updated_at", precision: 6, null: false
11    t.bigint "user_id", null: false
12    t.index ["user_id"], name: "index_companies_on_user_id"
13  end
14
15  create_table "users", force: :cascade do |t|
16    t.string "name"
17    t.string "address"
18    t.integer "age"
19    t.datetime "created_at", precision: 6, null: false
20    t.datetime "updated_at", precision: 6, null: false
21  end
22
23  add_foreign_key "companies", "users"
24end