As you are using the SupportMapFragment in a Fragment,you could call it like this:

SupportMapFragment mapFragment = Activity.SupportFragmentManager.FindFragmentById(Resource.Id.gmap).JavaCast<SupportMapFragment>();    
 if (mapFragment == null)
    {
     mapFragment = SupportMapFragment.NewInstance();
     mapFragment.GetMapAsync(this);
    }

refer the SupportMapFragment in your fragment xml:

<fragment xmlns_android="http://schemas.android.com/apk/res/android"
  xmlns_tools="http://schemas.android.com/tools"
  android_layout_width="match_parent"
  android_layout_height="match_parent"
  android_id="@+id/gmap"
  class="com.google.android.gms.maps.SupportMapFragment" />

update(like this,we could handle OnMapReady):

SupportMapFragment mapFragment = ChildFragmentManager.FindFragmentById(Resource.Id.gmap).JavaCast<SupportMapFragment>();    
mapFragment.GetMapAsync(this);

I have implemented a map into my application though on a single view. After creating a few layouts I wanted to implement a bottom navigation view which meant I had to use fragments. I have already moved my activity’s code into my fragment named map_fragment, at first I wasn’t sure where to use FindFragmentById but according to findViewById in Fragment this should be used on the OnCreateView() lifecycle method.
My main problem now is capturing the map fragment, since I get the error The name ‘SuportMapFragment’ does not exist in the current context when using

    var mapFragment = (SupportMapFragment)SupportFragmentManager.FindFragmentById(Resource.Id.gmap);
        // Was able to use FragmentManager, but it returns null...
    // var mapFragment = (SupportMapFragment)FragmentManager.FindFragmentById(Resource.Id.gmap);
        mapFragment.GetMapAsync(this);

Here’s the full code

    public class map_fragment : Android.Support.V4.App.Fragment, IOnMapReadyCallback
    {
        public override void OnCreate(Bundle savedInstanceState)
            {
            base.OnCreate(savedInstanceState);
                // Create your fragment here       
        }
        public override void OnActivityCreated(Bundle savedInstanceState)
        {
            base.OnActivityCreated(savedInstanceState);
            // Capture the Map Fragment
        }
        public override View OnCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
        {
            // Use this to return your custom view for this Fragment
            base.OnCreateView(inflater, container, savedInstanceState);

            View view = inflater.Inflate(Resource.Layout.activity_main, container, false);
            var mapFragment = (SupportMapFragment)SupportFragmentManager.FindFragmentById(Resource.Id.gmap);
            //var mapFragment = (SupportMapFragment)FragmentManager.FindFragmentById(Resource.Id.gmap);
            mapFragment.GetMapAsync(this);

            return view;
        }
        public void OnMapReady(GoogleMap _map)
        {
            // Some other code here
        }
}

As requested this is my main_activity

   public class MainActivity : AppCompatActivity
    {
        BottomNavigationView bottomNavigation;
        protected override void OnCreate(Bundle savedInstanceState)
        {
            base.OnCreate(savedInstanceState);
            // Initialize Xamarin Essentials for the Geolocation Service
            Xamarin.Essentials.Platform.Init(this, savedInstanceState);
            SetContentView(Resource.Layout.main_frame);
            //var toolbar = FindViewById<Android.Support.V7.Widget.Toolbar>(Resource.Id.toolbar);
            //if (toolbar != null)
            //{
            //    SetSupportActionBar(toolbar);
            //    SupportActionBar.SetDisplayHomeAsUpEnabled(false);
            //    SupportActionBar.SetHomeButtonEnabled(false);
            //}

            bottomNavigation = FindViewById<BottomNavigationView>(Resource.Id.bottom_navigation);

            bottomNavigation.NavigationItemSelected += BottomNavigation_NavigationItemSelected;

            // Load the first fragment on creation
            LoadFragment(Resource.Id.profile_view);

        }

        private void BottomNavigation_NavigationItemSelected(object sender, BottomNavigationView.NavigationItemSelectedEventArgs e)
        {
            LoadFragment(e.Item.ItemId);
        }

        void LoadFragment(int id)
        {
            Android.Support.V4.App.Fragment fragment = null;
            switch (id)
            {
                case Resource.Id.profile_view:
                    fragment = profile_fragment.NewInstance();
                    break;
                case Resource.Id.map_view:
                    fragment = map_fragment.NewInstance();
                    break;
            }

            if (fragment == null)
                return;

            SupportFragmentManager.BeginTransaction()
                .Replace(Resource.Id.content_frame, fragment)
                .Commit();
        }

        public override void OnRequestPermissionsResult(int requestCode, string[] permissions, Android.Content.PM.Permission[] grantResults)
        {
            Xamarin.Essentials.Platform.OnRequestPermissionsResult(requestCode, permissions, grantResults);

            base.OnRequestPermissionsResult(requestCode, permissions, grantResults);
        }
    }

These are the tutorials I am using as a guide

• https://devblogs.microsoft.com/xamarin/exploring-androids-bottom-navigation-view/
• https://devblogs.microsoft.com/xamarin/exploring-androids-bottom-navigation-view/