This is what happens over and over while trying to install Windows in a Bootcamp partition.
How can I get back to macOS?
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Hold down the control key and then select the circular arrow below the icon.
Note: On really old Mac models, you will only get a straight arrow when holding down the control key. If this happens, you will have to set the default startup operating system from the "Startup Disk" window in the System Preferences application.