[Dynamic 365]How can i know the key and the joining table Inside advance find function?

In "Developer resources" locate the web api url, it should look like this:

https://.../.../api/data/v9.0/

Copy this url, append the following, change the logical name, and paste it in the browser

https://.../.../api/data/v9.0/EntityDefinitions(LogicalName='put the name of the entity name that has the lookup column here')?$select=LogicalName&$expand=Attributes($filter=AttributeType eq Microsoft.Dynamics.CRM.AttributeTypeCode'Lookup')

Copy the result and format/beautify it using something like this, you shall easily find the lookup field information and it's referenced entity in the "Targets".

This information doesn't require any special permissions.

It's part of the entity metadata, which you can access by going to Settings > Customizations > Customize the system.

Select the entity on the list, then "N:1 Relationships" (or "1:N Relationships") and you will see the information about the field.

Alternatively, you can also go to Fields, then double click on the field name and you will see this info on a new window.

About the key used by the lookup, it's the primary key of the referenced entity (so here systemuserid)

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On a side note, I highly recommend using XRM toolbox for doing that (and a lot more things) faster, the plugin in this case is "Metadata Browser".