Vent/sink drain?

[Ryanneururer]

Is this pipe venting my toilet, shower drain, and sink? Also can I inset a wye to drain my sink into it? Also unsure what the current attachment is for and if it needs to stay on there. New construction basement finish. Thanks

 

[Cacher_Chick]

If there is no waste coming down the pipe from above, then it is a vent. The fitting is a cleanout, and it does need to exist, but can be moved higher or lower. A sanitary tee installed in the line would be the appropriate fitting for a lavatory connection.