How do I convert an HEIC file to jpg?

Dragging the thumbnail of the photo from Photos to the Desktop will work, but you will have no control over the quality. This will export the current preview as a JPEG. If you want to ensure a certain quality, use the "File > Export > Export ... photos" dialog and click the "v" to the right of "Photo Kind: JPEG". You will have more options.

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Drag the photo from your library to your desktop.

Indeed, that's correct. Dragging the photo to the desktop uses a fixed size and quality, which is full size and high quality should it help the OP.