The Express Edition is a SQL database engine with some limitations on CPU count and memory and missing SQL agent. It is free. I installed the version 2017 on some servers 7 years ago and they still run some supplier portal on it, that team was too lazy to even upgrade to newer versions.
MSSQL is extremely, extremely capable as a database engine.
But it also costs an arm and a leg. People who haven't used it don't know just how capable it is.