Audit all insert, delete, update and select statements on a schema in Oracle 12.2 I cannot understand how to create a Unified Audit policy in order to audit all INSERT, UPDATE, DELETE and SELECT statements performed on all the objects within a specific schema.

The goal is to track the tables and views involved in a specific PL/SQL procedure for a reverse engineering task.

For what I understood from the documentation, the only option is to specify each table/view to track within the policy. Is there an "audit all objects within a schema" construct for the create audit policy statement?
You described traditional auditing. For unified auditing the parameter audit_trail has no meaning and is actually not considered by Oracle. As mentioned earlier, with unified auditing you have to create an audit policy.
Oracle does not write to SYS.AUD$ with unified auditing. It writes to audsys.aud$unified or spillover files on the OS (if the DB is not accessible). Your data is visible through the view unified_audit_trail, which selects from audsys.aud$unified and v$unified_audit_trail (spillover files).
To audit all insert,update,delete,select on a table scott.t1 you would use something like
create audit policy T1_AUDIT  
       ACTIONS select on scott.t1,  
               insert on scott.t1,  
               update on scott.t1,  
               delete on scott.t1;  
  
audit policy T1_AUDIT;