No. You just cut down the fully herbaceous ones. You will recognise these by the foliage dying back naturally. You are only cutting down to keep a tidy garden: It is better for the garden to let the foliage die back on its own and decay on the ground.

No, not all of them. You need to be judicious. Generally, herbaceous plants should be allowed to die down with the frosts. These can either be allowed to stay on the beds, giving the crowns a bit of protection, or cut back once they've died off.

On the other hand, some perennials remain green....Penstemon for example. I leave mine all year and just tidy the plants up in spring

I like to leave cutting back many of my perennials until early spring.

There are several reasons for that

1. Many have seed heads which feed the birds over winter

2. If you cut back hard in autumn you're left with an "empty" garden

3. Leaving foliage on the plant provides cover for over wintering bugs/small mammals etc

4. The frost on some dead but vertical foliage or spent flower buds can look lovely.

5. For the slightly more tender perennials leaving the old foliage on provides a degree of protection from frosts.

As others have said some perennials don't die back or only this years stems do and small new shoots which will be next years growth are already there in autumn.

One example of this is some of the perennial rudbeckias where the new small foliage is evergreen in all but the hardest of winters

Many thanks for your help I will put the secatuers away