Oh waow, I was not even aware that multiple places were possible for those CF Alternate Layouts :)

In "Step 5 - create an Alternate Layout for the Custom Field" of https://magazine.joomla.org/all-issues/october-2021/custom-fields-episode-7-part-1-one-custom-field-to-rule-them-all I explain the 2 methods that I knew about, namely using the front-end template.

BTW, I had never thought of using CF in the backend. Where / how is it available?

1. the "manual" way

Create the following file
/templates/[my_template]/html/layouts/com_fields/field/my-own-layout.php
(note: underscores are not allowed in the name of that file)
where you paste the content of the following file
/components/com_fields/layouts/field/render.php
which is actually the default layout used to render Custom Fields

2. via Joomla's interface

• go to System > Site Templates > Cassiopeia Details and Files > Create Overrides > Layouts > com_fields
• click on "field"
• you get the following confirmation message: "Override created in /templates/cassiopeia/html/layouts/com_fields/field"
• rename that file

doh

I didnt read properly and didnt notice that the layout is only for the rendering of the field output. I was thinking I could have a layout for the display of the form input

I also find it very confusing.

I assumed that you can create and select an override here for the output of this particular field.

For example, I wanted to create an override of plg_fields_subform. By default this is output as a list, which I want to change. When I give a custom name (mysubform.php) to this override, I can't select this anywhere. It must remain the name subform.php, so that my override is accepted.

However, if I want to use different subforms, then I can't assign my override specifically. It applies to all subforms and causes errors with subforms that do not match my override.

[EDIT]
I finished it with the override of "com_fields/field/render.php" after all.

In my case I made it easier then. I copied the file "subform.php" with my changes into the directory "templates/cassiopeia/html/layouts/com_fields/field/" and renamed it.

Before the foreach loop starts, I had to adjust the variables and add FieldsHelper.

<?php
defined('_JEXEC') or die;

use Joomla\CMS\Language\Text;
use Joomla\Component\Fields\Administrator\Helper\FieldsHelper;

if (!array_key_exists('field', $displayData))
{
	return;
}

$result = '';
$buttons = '';
$context = '';
$field = $displayData['field'];

if (empty($field->subform_rows))
{
	return;
}

foreach ($field->subform_rows as $subform_row){
...
}

?>

<?php if (trim($result) != '') : ?>
	<p><?php echo $buttons; ?></p>
	<?php echo $result; ?>
<?php endif; ?>

HI @LukasHH
Have a look at https://magazine.joomla.org/all-issues/october-2021/custom-fields-episode-7-part-1-one-custom-field-to-rule-them-all and in particular at this screenshot :
https://magazine.joomla.org/images/2021/october/cf-ep7/cf-ep7-7-alternate-layout.png

It is directly in the interface of any given Custom Field that you can select the Alternate Layout that you created.

@woluweb

Yes - I took a closer look at it once and tried it out afterwards. I have described my solution in my comment under [Edit].