Can I make a private layer which I created, public?

Sharing a layer publicly once you have created it is possible. Here's how.

Once you have created and edited a layer, you may want to share it.

Locate the layer in the layers panel, and right-click on it. 

In this example I am using the layer, DMBoardLocal2019:

There are several editing options available to you.

Choose "Edit this layer".

Use the dropdown box and choose Public in place of Private.