SQL Relationships And Identities

Can anyone tell me what is best practices to define relationships between
tables and to define their primary key?

For example...
One table is Orders and other is OrderDetails.
For ease of accessing datas from table OrderDetails I have fields copied
from Orders like year_of_order, order_number, client_id.

Now, is it better to link those tables on this three fields, because they
are already there or define new field in OrderDetails with name OrderID and
link it on ID field from table Orders?
In this case I will always access table Orders to check year of order,
order number or client id for records in OrderDetails.

Which one of these examples have better performace on server?

I'm also doubting is it good to define field Id in each table and use it
for relationships to other tables or link tables on fields like
Number_of_order.

Is it enough to define field "Id" set it as PK and Identity Seed 1 for
unique mark of record and preserving consistency of data.

Is it good practice to link tables on primary key from one table to other
column in other table?

Thnks.