Xamarin forms: How to make tabs on bottom of the page?

c# code worked it.

using System;
using Xamarin.Forms.PlatformConfiguration.AndroidSpecific;
using Xamarin.Forms;

namespace YouProjectNameSpace
{
    public partial class MyTabbedPage : TabbedPage
    {
        public MyTabbedPage()
        {
            InitializeComponent();
            On<Xamarin.Forms.PlatformConfiguration.Android>().SetToolbarPlacement(ToolbarPlacement.Bottom);
        }
    }
}

Edit: for new Xamarin.Forms projects you should use Xamarin.Forms Shell, it provides easy tabs and other great features.

To accomplish that you have 3 options:

1) To use new bottom tabbar native control you must have Xamarin Forms version 3.1 or above.

On your tabbed page you need to add the android specification for bottom placemente:

XAML

<?xml version="1.0" encoding="utf-8"?>
<TabbedPage xmlns="http://xamarin.com/schemas/2014/forms"
    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
    xmlns:android="clr-namespace:Xamarin.Forms.PlatformConfiguration.AndroidSpecific;assembly=Xamarin.Forms.Core"
    android:TabbedPage.ToolbarPlacement="Bottom"
    x:Class="YouProjectNameSpace.MyTabbedPage">
</TabbedPage>

Or c# code behind

using System;
using Xamarin.Forms.PlatformConfiguration;
using Xamarin.Forms.PlatformConfiguration.AndroidSpecific;
using Xamarin.Forms;

namespace YouProjectNameSpace
{
    public partial class MyTabbedPage : TabbedPage
    {
        public MyTabbedPage()
        {
            InitializeComponent();
            On<Android>().SetToolbarPlacement(ToolbarPlacement.Bottom);
        }
    }
}

If you want more customization you can add:

<?xml version="1.0" encoding="utf-8"?>
<TabbedPage xmlns="http://xamarin.com/schemas/2014/forms"
    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
    xmlns:android="clr-namespace:Xamarin.Forms.PlatformConfiguration.AndroidSpecific;assembly=Xamarin.Forms.Core"
    android:TabbedPage.ToolbarPlacement="Bottom"
    BarBackgroundColor="#2196F3"
    BarTextColor="White"
    android:TabbedPage.BarItemColor="#66FFFFFF"
    android:TabbedPage.BarSelectedItemColor="White"
    x:Class="YouProjectNameSpace.MyTabbedPage">
</TabbedPage>

2) Create a custom renderer for Android of the tabbed page and move it to the bottom

using System;
using Xamarin.Forms;

namespace YouProjectNameSpace
{
    public class CustomTabbedPage : TabbedPage
    {
    }
}

And the renderer:

using System;
using Android.Content;
using Android.Support.Design.Widget;
using Android.Support.V4.View;
using Xamarin.Forms;
using Xamarin.Forms.Platform.Android.AppCompat;

[assembly: ExportRenderer(typeof(CustomTabbedPage), typeof(CustomTabbedPageRenderer))]
namespace YouProjectNameSpace.Android
{
    public class CustomTabbedPageRenderer : TabbedPageRenderer
    {
        public CustomTabbedPageRenderer(Context context): base(context)
        {
        }

        protected override void OnLayout(bool changed, int l, int t, int r, int b)
        {
            this.InvertLayoutThroughScale();

            base.OnLayout(changed, l, t, r, b);
        }

        private void InvertLayoutThroughScale()
        {
            this.ViewGroup.ScaleY = -1;

            TabLayout tabLayout = null;
            ViewPager viewPager = null;

            for (int i = 0; i < ChildCount; ++i)
            {
                Android.Views.View view = GetChildAt(i);
                if (view is TabLayout tabs)
                    tabLayout = tabs;
                else if (view is ViewPager pager)
                    viewPager = pager;
            }

            tabLayout.ViewTreeObserver.AddOnGlobalLayoutListener(new GlobalLayoutListener(viewPager, tabLayout));

            tabLayout.ScaleY = viewPager.ScaleY = -1;
        }

        private class GlobalLayoutListener : Java.Lang.Object, Android.Views.ViewTreeObserver.IOnGlobalLayoutListener
        {
            private readonly ViewPager viewPager;
            private readonly TabLayout tabLayout;

            public GlobalLayoutListener(ViewPager viewPager, TabLayout tabLayout)
            {
                this.viewPager = viewPager;
                this.tabLayout = tabLayout;
            }

            public void OnGlobalLayout()
            {
                this.viewPager.SetPadding(0, -this.tabLayout.MeasuredHeight, 0, 0);
                this.tabLayout.ViewTreeObserver.RemoveOnGlobalLayoutListener(this);
            }
        }
    }
}

3) Use specific library like PXTabs, this one creates a full Xamarin Forms bottom tabs without any native code.

If you want to read more about bottom tabs and renderers:

Setting TabbedPage Toolbar Placement and Color

Xamarin.Forms: Official Bottom Navigation/Bottom Tabs on Android

BottomNavigationBarXF