Currently we have a server on Oracle version 18. We are planning to migrate to MariaDB. Is there any free service provided by MariaDB or oracle that helps migrate a database from Oracle to MariaDB?
There is certainly no such service by Oracle as they wouldn't want anyone to migrate away from their own database system.
MariaDB Corporation has a paid migration service to help migrating to MariaDB. This is run by experts in the field.
AWS has a fully automated service that appears to be able to migrate your schema and data, but presumably not much beyond that like stored procedures, functions and triggers. AWS is not your only option, though - other tools for this exist.
There are many articles around the web on how to do it yourself, but this will require a lot of knowledge and skills. See e.g. this article at Several Nines for a starter.