Dimensions and metrics
Dimensions are green and describe things — country, campaign, date, page path. Metrics are blue and measure them — sessions, revenue, clicks.
A chart is essentially a choice of dimensions to break down by and metrics to display. Get that pairing right and the chart usually follows.
Types and aggregation
Each field has a type (text, number, date, currency, geo, URL) and metrics have a default aggregation (sum, average, count, count distinct, min, max).
Edit them in the data source, not per chart. Fixing the currency format once in the data source beats fixing it on nine charts.
An order ID or postcode arrives as a number, so Looker Studio makes it a metric and cheerfully sums it. Change the type to text and its role to dimension in the data source.
Changing a field is not a rename
Changing a field's aggregation in a chart affects only that chart. Changing it in the data source affects everything using it. When something looks wrong in several places, the fix belongs in the data source.
Data freshness
Looker Studio caches results. Resource → Manage data sources → Data freshness sets how long, and the options depend on the connector — 15 minutes for BigQuery, 12 hours for some others.
The Refresh data button in the toolbar clears the cache for the current report immediately, which is the answer when someone says the numbers look stale.
Shorter freshness means more queries. On BigQuery that is a direct cost, so match the interval to how often the data genuinely changes rather than setting it to the minimum.