- follow https://laravel.com/docs/master/eloquent-relationships#many-to-many-polymorphic-relations as usual
 - thanx to setup oneToMany relation between 
tag & taggable 
under taggable model add
<?php
class Taggable extends Model
{
    protected $with = ['related'];
    public $timestamps = false;
    /* -------------------------------------------------------------------------- */
    /*                                  RELATIONS                                 */
    /* -------------------------------------------------------------------------- */
    public function tag()
    {
        return $this->belongsTo(Tag::class);
    }
    public function related()
    {
        return $this->morphTo();
    }
}under tag model add
<?php
class Tag extends Model
{
    protected $with = ['taggables'];
    public $timestamps = false;
    /* -------------------------------------------------------------------------- */
    /*                                  RELATIONS                                 */
    /* -------------------------------------------------------------------------- */
    public function taggables()
    {
        return $this->hasMany(Taggable::class);
    }
    
    /* -------------------------------------------------------------------------- */
    /*                                  ACCESSORS                                 */
    /* -------------------------------------------------------------------------- */
    public function getItemsAttribute()
    {
        return $this->taggables
            ->groupBy('taggable_type')
            ->map(function ($v, $k) {
                return app($k)->whereIn('id', $v->pluck('taggable_id'))->get();
            })->all();
    }
}and now use it like
Tag::first()->items;
update
relatedmethod according to docs as with plain$this->morphTo()we would getnullinstead