if you run php artisan vendor:publish and then choose laravel-mail
Then in resources/views/vendor/mail/html you will see a header.blade.php file.
which contains
@if (trim($slot) === 'Laravel')
<img src="https://laravel.com/img/notification-logo.png" class="logo" alt="Laravel Logo">
@else
{{ $slot }}
@endif
which says if the header slot contains 'laravel' then show the laravel logo, else, whatever is in the slot.
At the top of message.blade.php you will see
@component('mail::layout')
{{-- Header --}}
@slot('header')
@component('mail::header', ['url' => config('app.url')])
{{ config('app.name') }}
@endcomponent
@endslot
This is where the header is used, and in the slot is {{ config('app.name') }} , ie, your app name from the config.
You can replace this line in the message.blade.php file to include your own logo and/or product name.