When you click on Edit/Emoji and Symbols, you get a display of various symbols. There are some arrow selections among them. However, they can only be added to a title. So, if you drag a title and overlay it above your sports clip, you can click on one of the symbols and it will automatically appear in the title. However, that will not do you much good, as you will not be able to adjust the position of the arrow or animate it so that it follows the player.
The way you can insert an arrow into your video is to create a transparency screen (see below) in the Preview app on the Mac, insert an arrow into it, and save to your desktop. Then import the arrow transparency that you created into iMovie and overlay it onto your sports clip. Then use the Picture in Picture feature to insert the arrow into the clip.

You then can animate the arrow with the Keystrokes feature of PiP so that it follows the player. See this Help menu link that describes how to use the Picture in Picture feature and animate it.
https://help.apple.com/imovie/mac/10.1/#/mova1aaa682b
Creating the transparency screen is easy. Take a duplicate of any random photo and click on it to open it in Preview. Then, in the Preview menu, do Edit/Select All, Edit/ Delete. The picture will disappear leaving you with a blank transparency screen. Then go to Tool/Annotate/Arrow and insert an arrow into the transparency screen. You can use the tool bar at the top of the Preview screen to set the color and size of the arrow. Also, there are drag handles that let you adjust the position and configuration of the arrow. When done, do a File/Save to your desktop. Then import the transparency arrow that you created into iMovie. Insert it above your sports clip and select the arrow clip. Click on the overlay tool (the one that looks like overlapping rectangles) in the tool bar at the upper right of your screen. Click on Picture in Picture in the selection box that appears. Adjust the position of the arrow, and the size of it by dragging the corners of the outline of the arrow photo. Then click on the blue reset button to apply your adjustments. Read the link that I inserted above for a more detailed explanation of how to use the PiP feature and how to animate the arrow.
-- Rich