All databases ALWAYS run in "dedicated" mode. It is not really even a "mode", it is a method of connecting.

So, you always have dedicated server connections (you need them to start up and shut down and perform other administrative operations). Dedicated server is NOT optional, it is a fact.

Shared server is configured via network settings and init.ora settings. here is the database setup step by steps:

https://docs.oracle.com/en/database/oracle/oracle-database/19/admin/managing-processes.html#GUID-CCC18A52-E47B-48AA-8811-4E87023DB99D

then, when you connect, you would either use a tns connect string that connects to a share server only connection OR use "server=shared" in the connection string, for example:

ORCL_DS =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = localhost)(PORT = 1521))
    (CONNECT_DATA =<b>
      (SERVER = DEDICATED)</b>
      (SERVICE_NAME = xxxxx)
    )
  )

ORCL_SS =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = localhost)(PORT = 1521))
    (CONNECT_DATA =<b>
      (SERVER = SHARED)</b>
      (SERVICE_NAME = yyyy)
    )
  )