Help clean up my reporting data in SQL and dbt

I have some messy sales and traffic data and I need help to make it more reliable. The job is to review a few SQL queries, improve one small dbt model, and help me understand why some numbers are changing from week to week. It is not a full data platform project. I use BigQuery and a simple Power BI report, so I need someone who can explain things in plain English and leave the queries easy to maintain. I think this is a small but important fix, maybe a few hours to start, then more if it goes well.