Hi @Madeleine_Capshaw_MA and welcome to the community!
A lookup field only works between linked fields in the same base. So that won’t be possible.
You can’t hide tables, only restrict the creation and deleting of records.
I think you’ll need an external party (eg Zapier) for this, as automations also only work within the same base and you can’t sync back an already synced table.
Hi,
You may create separate table in Billing for output, link it to main table, reflect attachments there via lookup, and then sync it back to another table of Inventory. sometimes, even more than 2 syncs are needed between 2 bases, including multi-source (from different tables, with field mapping) and multi-destination (when one of your bases receive 3rd party data by API, and that data needs to be shared to other bases).
Of course, it depends on plan limit.