Check $ORACLE_HOME/network/admin and there would be a sample file to configure your listener. The listener works on the server side (i.e on the server where your instance resides) the clients should have tnsnames.ora file under the same directory on the client side. The tnsnames.ora would have the entry on the service information. This again you can find under your client's $ORACLE_HOME/network/admin/samples directory.
BTW, you could modify them and then would have it under $ORACLE_HOME/network/admin directory.
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