Setup Facebook Pixel via GTM using GA4 Data Layer Schema

<!-- Facebook Pixel Code -->

<script>

  !function(f,b,e,v,n,t,s)

  {if(f.fbq)return;n=f.fbq=function(){n.callMethod?

  n.callMethod.apply(n,arguments):n.queue.push(arguments)};

  if(!f._fbq)f._fbq=n;n.push=n;n.loaded=!0;n.version='2.0';

  n.queue=[];t=b.createElement(e);t.async=!0;

  t.src=v;s=b.getElementsByTagName(e)[0];

  s.parentNode.insertBefore(t,s)}(window, document,'script',

  'https://connect.facebook.net/en_US/fbevents.js');

  fbq('init', '805025110181864');

</script>

<!-- End Facebook Pixel Code -->

<script>

  fbq('track', 'PageView');

</script>

<noscript><img height="1" width="1" style="display:none"

src="https://www.facebook.com/tr?id=805025110181864&ev=PageView&noscript=1"

/></noscript>

1. View Content Event: 

<script>

    fbq('track','ViewContent',{

    content_type: 'product',

    content_ids: {{dlv - fb - product id}},

    content_name: {{dlv – fb - product name}},

    value: {{dlv – fb - product price}},

    currency: 'BDT'

    });

</script>

Create Data Layer Variables (Name - Data Layer Variable Name) :

dlv - fb - product id : ecommerce.items.0.item_id

dlv – fb - product price : ecommerce.items.0.price

dlv – fb - product name : ecommerce.items.0.item_name

Trigger:

Event equals: view_item

2. Add to cart Event

<script>

    fbq('track','AddToCart',{

    content_type: 'product',

    content_ids: {{dlv - fb - product id}},

    content_name: {{dlv – fb - product name}},

    value: {{dlv – fb - product price}},

    currency: 'BDT'

    });

</script>

Create Data Layer Variables (Name - Data Layer Variable Name) :

dlv - fb - product id : ecommerce.items.0.item_id

dlv – fb - product price : ecommerce.items.0.price

dlv – fb - product name : ecommerce.items.0.item_name

Trigger:

Event equals: add_to_cart

3. Purchase Event: 

Step - 01: Create Data layer variable for pushing Purchase value

Name:

dlv - FB - Purchase value  

ecommerce.purchase.actionField.revenue
or 

ecommerce.value

Step - 02: Create Data Layer Variable for Products (Multiple items)

Name:

dlv - FB - Purchase Product
ecommerce.items 

Step - 03: Create Custom JavaScript Variable to push multiple products info from Array 

Name: 

jsv - FB - Purchase Product - AR

Code: 

function() {

 var items = {{dlv - FB - Purchase Product}};

 return items.map(function(items) { return items.item_id; }).join();

}

Step - 04: Create Trigger for purchase event

Event equals: purchase

Step -05: Create tag and finally test from preview mode

Code 1: 

<script>

    fbq('track', 'Purchase', {

    value: {{dlv - FB - Purchase value}},

    currency: 'BDT',

    content_type: 'product',

    content_ids: {{jsv - FB - Purchase Product - AR}},

    });

</script>

Code 2: 

<script>

    fbq('track', 'Purchase', {

    value: {{dlv - FB - Purchase value}},

    currency: 'BDT',

event_id: {{FB - Event ID}},

    content_type: 'product',

    content_ids: {{jsv - FB - Purchase Product - AR}},

    });

</script>

Source: Combine GTM Product IDs for Facebook Conversion Tags on Magento | Elevar (getelevar.com)