BigQuery has a number of publicly available datasets that you can use to play around with, or to build and train data models. The storage for these is free, that is, paid for by Google, so you only have to pay for queries that you run against these datasets. These queries are charged like all other queries.
In the following examples, we will make use of such public datasets, which can be accessed from within queries using notation such as bigquery-public-data.samples.natality.