Maximum number & size of fields (nested) inside a document

I have an ES setup that has an index called customer with details of the customers (user_id, name,age etc.) and I have created a separate index called transaction which has details of transactions (user_id,date,item list (containing sku etc) etc.)
I need to find the users who have purchased an item (sku) and who are gender female for example.
Is there a way to join these two indices?
If not, and I decide to add the transactions for each user as a nested object inside the customer mapping, what is the maximum number of fields (including nested objects) can each document have? And is there a maximum number of objects inside each item of the nested objects?
The customer will have many fields such as vouchers, communications, offers, 1000s of transactions etc.

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