Explanation
The Delete permission is missing on the user for Accounts is what is preventing the user from completing the merge. Merging accounts is a way to combine up to three duplicate accounts into one master record and update the related records accordingly. Merging accounts requires the Delete permission on accounts because merging accounts involves deleting the duplicate records and keeping only the master record.

Explanation
Bulk API and Dataloader.io are both tools that can be used to update large numbers of records in Salesforce.
Bulk API is a RESTful API that allows you to create, update, delete, or query millions of records asynchronously by submitting batches of data. Dataloader.io is a web-based data loader tool that allows you to import, export, or delete data using a simple user interface. References:
https://developer.salesforce.com/docs/atlas.en-us.api_asynch.meta/api_asynch/asynch_api_intro.htm
https://dataloader.io/

Explanation
A lookup relationship is a type of field that links one object to another object. It allows you to access related data from another object and display it on a record page. A lookup relationship can be used to link the custom object that stores financial information to the Account object, so that only finance team members can see the related financial information about the account. References:
https://help.salesforce.com/s/articleView?id=sf.relationships_lookup.htm&type=5

Explanation
A lightning action is a type of quick action that can invoke a Lightning component, a Visualforce page, or a URL. The administrator can use a lightning action to redirect the user to a different page after they finish the screen flow. This way, the user will not see the initial screen again and will not misuse the flow tool.
References:
https://help.salesforce.com/s/articleView?id=sf.lightning_component_actions.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.flow_distribute_lex.htm&type=5