There’s a couple of ways of getting your Sketchup Make files into Sketchup Free. You could use a Trimble Connect account. You’d need to publish your model from Sketchup Make to Trimble Connect first.
The other way doesn’t require using Trimble Connect. Go to the folder icon at the top left, then choose “Insert”.
Then either click the icon on the right that looks like a laptop and choose the file to upload. You can also drag the file to here from your computer. The other icon lets you insert from Trimble Connect.
Once you’ve dragged or selected a file, the filename will appear and you then click “Component”.
Then click to place the component in your model. Your file will of course not be loaded, but inserted into the current file. If you start with a new file in Free, then insert your file, you could work from there. It comes in as a Component but you could explode it if you want.