Say I have two simple tables Users and Posts defined as the following:

CREATE TABLE [dbo].[Users](
    [UserID] [int] IDENTITY(1,1) NOT NULL,
    [Username] [varchar](255) NULL,
    [FirstName] [varchar](255) NULL,
    [LastName] [varchar](255) NULL,
    [DateOfBirth] [datetime] NULL,
    [Age] [int] NULL,
CONSTRAINT [PK_Users] PRIMARY KEY CLUSTERED 
(
    [UserID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]

GO

CREATE TABLE [dbo].[Posts](
    [PostID] [int] IDENTITY(1,1) NOT NULL,
    [Content] [varchar](max) NULL,
    [NumberOfLikes] [int] NULL,
    [UserID] [int] NULL,
    [CreateDateUTC] [datetime] NULL,
    [Tags] [varchar](max) NULL,
 CONSTRAINT [PK_Posts] PRIMARY KEY CLUSTERED 
(
    [PostID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]

GO

Obviously when the database gets very large, performance tuning will become necessary. I figured that the column UserID in Posts is very essential because most of my queries filter by it. So I figured that I'll need to define an INDEX on that column. As for the covering fields, let's assume, for the purpose of this question, that all of my queries look the same (except for the WHERE part):

SELECT
  Posts.Content
 ,Posts.NumberOfLikes
 ,Users.UserName
FROM
  Posts
INNER JOIN
  Users
    ON
    Posts.UserID = Users.UserID
WHERE
  Posts.UserID = @UserID;

My question is about the covering fields. I can easily define an index that covers Content and NumberOfLikes like this:

CREATE NONCLUSTERED INDEX [IX_Posts_UserID] ON [dbo].[Posts] (UserID) INCLUDE (Content, NumberOfLikes)

However, my query always joins with the Users table. So is my index still relevant (in terms of performance) despite that my query includes more fields (from another table) than the covering fields of the index? I know that I cannot cover fields from another table so how do I optimize the query in this case? When I looked at the execution plan, I saw that my index IX_Posts_UserID was in fact used (50%, and another 50% by PK_Users ) but I was puzzled as to how that happened since I'm selecting columns that are not covered by the index.

So the ultimate question here is: how do table joins factor in the decision of whether indexes are used by SQL Server? Or even simpler, how do joins affect indexes?

EDIT: Per Simonare's comment, below is the execution plan: