The item ID in the tags must match the item ID in product feeds, or Google (or Facebook, AWIN, etc.) won't be able to match the conversions (or remarketing events) with products previously submitted.
Some common Item ID formats:
shopify_XX_{product_id}_{variant_id}. XX is a 2-letter country code. This format is used by the Google Youtube channel app.
{product_id}_{variant_id}
{variant_id}. This format is used by the Meta channel app
{sku}
For Google Ads remarketing tags, the item ID in tag has another option which is to match against item group ID in product feeds.
Most feed apps use the product ID as the item group ID as in Shopify, variants of a product is a native equivalence to the item groups in Google Shopping. Because of this, the item ID in the remarketing tags could be set to use Product ID.
{product_id}
The item id can be configured as a template. By interleaving the texts and the tokens, the IDs will be composed at runtime to form the firing item ID.
The supported tokens include {product_id}, {variant_id} and {sku}
