SharePlex processes do execute DML on the source database but such activity is only confined to data manipulation of SharePlex internal tables. These internal tables are created when the ora_setup utility is run after installation of SharePlex. When SharePlex is upgraded, the ora_setup is re-run so that the internal tables are updated as needed as newer features may involve additional internal tables and/or additional characteristics in existing tables. The internal tables form an essential part of SharePlex replication as they control various functionalities of SharePlex.
SharePlex never tempers with any other object in the source database.
On target it is a different story. SharePlex do apply the changes it gathers from the source database to the target database. In fact SharePlex is the only user which manipulates data on the target database. The other users are only supposed to access the target database for reading only.
The above discussion only pertains to one way replication as it is possible to configure SharePlex to replicate in a peer to peer manner. In that case all SharePlex instances forming part of replication can assume the role of both source and target at the same time or at different times. So SharePlex can very much apply DML on all the nodes involved in SharePlex replication.