If both BigQuery and Spanner are relational database, and all can Horizontally scale.
How should I choose one of them to use?
Does BigQuery analytics enginee also support Spanner?
At a very high level, BigQuery is an analytics database, and Spanner is a transactional database. That’s the main difference, so it really depends on your use case. The best I can advise is to study the product docs to determine which one will work best for you.
Hope this helps!