You need to get clarity on whether there is or is not a relationship (lookup/master-detail) with the
Partners__c object. You can't write a query without knowing your data model. You can review the objects in your Setup interface or use Schema Builder, and for the purpose of writing subqueries, make sure to look at the definition of any relationship fields (which are located on the child object) to get the relationship name.
As you've found, there's two ways you can associate your Accounts with your Partners in the context of this query. Both assume there is a lookup relationship (
Account. However, your query is confused on a couple of points.
SELECT .. FROM Partners subquery is the route by which you would pull information about the Partner associated with each Account in your query results. The relationship name (
Partners, here) is defined on the lookup field, although
Partners is not a legal name for a custom relationship. The
WHERE clause on this subquery does not make sense, because a subquery like this one is already limited based upon the lookup relationship between the objects (i.e., no filtration by matching names is required). For more information, read through Relationship Queries
The other tool you use a semi-join, which you do in the
WHERE portion of your query. A semi-join can limit query results based on the results of another query against a related object, like your query against
Partners__c. However, your sub-select does not make sense. You appear to be matching
Account.Name against a field (
partner_Account_ID__c) that contains Id values, so you should do
WHERE Id IN. Additionally,
partnername is not a legal custom field name.
Based on your statement of your objectives, it sounds like you probably do need to perform a semi-join, but you may not need the relationship subquery. You'd need to be more specific about exactly which data points you need to obtain.
Lastly, please note that 'junction object' has a specific meaning in Salesforce; a junction object is the detail on two master-detail relationships to different objects, representing a many-to-many relationship.
If you're struggling with SOQL, I recommend reading through the entire SOQL reference and the Write SOQL Queries unit on Trailhead. But the first step is getting a clear understanding of your own data model - you simply cannot form queries if you don't have this.