I wanted to take items for a multi choice type column and convert it to a string for a single line field in a SharePoint column

First I added Initialize variable and gave it the type of Array. Put whatever you want for the name

Then I created an item called Append to array variable. On the value part select the dynamic content for your multiple choice field. This will create an apply to each item which is normal like so:

After that create an item called Join and in the from spot put your variable from earlier then under join with enter what you want to divide the items with (example: ,)

Once all done here is what my input and output looks like:

Reference: https://debajmecrm.com/convert-an-array-to-string-in-power-automate-microsoft-flow/