Q1. Comments by the Community User
The Stack Exchange staff has updated review queues, according to which in case of providing feedback for first posts (questions and answers) reviewers can leave standardised feedback comments that appear from the community account:
Reviewers are encouraged to leave comments for the post-author, but now they can also choose from a few options of canned feedback which will appear as a comment from the Community account.
So as per the new update, in reality the comments are by real users only, but it just appears with the name of the community user.
Q2. Votes by the Community User
As per the description of the community user, it:
Owns up/down votes from very active deleted users.
That is to say those votes casted by deleted users, which as per the discretion of the staff, should be retained considering their formerly active participation, appear as votes under the name of the Community User. So we would’ve had some formerly active users who were liberal with votes. Their votes are the ones that appear under the community account.