The slide to answer appears on any call when the phone is locked. Click the side button once to silence or twice to decline.

It means there’s a focus mode active. It’s been asked so many times. I literally just googled it and it was the first result I find it hard to believe you couldn’t find anything about it. You can change focus mode in control centre.

You might have restrictions enabled somehow. Check your screen time settings then scroll down to the restrictions settings. Could also be because of country and region settings