Where does the actual data stored by add_post_meta

Yes, we can use existing tables in WordPress for storing the values.
Post meta fields are stored in the {$wpdb->prefix}_postmeta table (depending on the table prefix; wp_postmeta by default). If the meta key "like_count" is already present in the table along with the post ID then update_post_meta() will update this, and otherwise this function will insert a new row with this key.

For more reference you can check here : https://codex.wordpress.org/Function_Reference/update_post_meta

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