Lookup Columns can be modified with filters to provide cascading lookups and Data Lookup Columns to provide cross site lookups.
Creating Cascading and Cross Site Lookups
Provide more logical within your lookup columns by using dynamic filters, and provide cross site lookup columns.
Lookup Columns within SharePoint list forms can be customized allowing you to add dynamic filters based upon other form fields, expressions, or static values.
Data Lookups can also be used as a replacement of the out-of-the-box SharePoint lookup column to provide cross site lookups.
Use the videos and documentation below to learn how to use lookups within your SharePoint List Forms.