The key for an ordinal to belong to a collection is that it…

The key for an ordinal to belong to a collection is that its metadata, specifically the subTypeData field, must contain the collectionId and subType must be collectionItem. All items in a collection must share the same Collection ID, which is the ordinal used as the collection's Cover.
Here's a link with more documentation. FYI, the collection's Cover is also an ordinal, but its metadata in subType is set as "collection" instead of "collectionItem".
Important: All ordinal metadata is placed after an Op Return in the ordinal inscription transaction.
If you deem it necessary, we can schedule a remote session to explain it to you.
https://docs.1satordinals.com/adding-metadata/collectionitem-subtype