Looking at the source code, we can see

<?php

namespace Illuminate\Database\Eloquent;

use LogicException;
use Illuminate\Support\Arr;
use Illuminate\Contracts\Queue\QueueableCollection;
use Illuminate\Support\Collection as BaseCollection;

class Collection extends BaseCollection implements QueueableCollection

In other words, a subclass of IlluminateDatabaseEloquentCollection是IlluminateSupportCollection.

The methods you mentioned are IlluminateDatabaseEloquentCollection中是这样定义的，以pluckfor example.

/**
 * Get an array with the values of a given key.
 *
 * @param  string  $value
 * @param  string|null  $key
 * @return \Illuminate\Support\Collection
 */
public function pluck($value, $key = null)
{
    return $this->toBase()->pluck($value, $key);
}

And how is the definition used in toBase函数在IlluminateDatabaseEloquentCollection中没有定义，而是在IlluminateSupportCollection中定义了。那么在子类中没有重写的方法，就会调用父类的方法。我们看看toBase在IlluminateSupportCollection used here.

/**
 * Get a base Support collection instance from this collection.
 *
 * @return \Illuminate\Support\Collection
 */
public function toBase()
{
    return new self($this);
}

Look, it’s returnednew self($this)，一个新的实例。由于这是在父类中的，自然返回的实例是IlluminateSupportCollection了。IlluminateSupportCollection中的pluckThe definition is like this.

/**
 * Get the values of a given key.
 *
 * @param  string|array  $value
 * @param  string|null  $key
 * @return static
 */
public function pluck($value, $key = null)
{
    return new static(Arr::pluck($this->items, $value, $key));
}

And the definition of IlluminateSupportArr中pluck is this.

/**
 * Pluck an array of values from an array.
 *
 * @param  array  $array
 * @param  string|array  $value
 * @param  string|array|null  $key
 * @return array
 */
public static function pluck($array, $value, $key = null);

What is returned is an array.
The difference between this IlluminateSupportCollection中的new static(Arr::pluck)，意思就是新建一个类的实例（new self和new static can be found at https://www.laravist.com/blog/post/php-new-static-and-new-self).

How about it, do you understand now?