Just reporting back to say that my iMac is now running off a Samsung T7 SSD. This was the first time in using a Mac for more than twenty-five years that I had had a failing hard drive—hence the elementary questions. Thank you to everyone who provided advice.
I had hoped that the problem with an annoying popup window asking where the Notification Centre is would disappear with the fresh installation, but it hasn’t. In addition, I have the Dock set to automatically hide and show, but recently it had been showing at startup, and this behaviour has also persisted.