So a secondary school English teacher in his or her first year might have a couple of Y8 middle sets, Y10 bottom set, Y11 top set, a Y12 or 13 class (A level), a Y7 top set, a Y9 middle set. In addition the teacher might get roped in to teach one or two lessons in another subject not their specialism. RE for example. A physics teacher might be asked to take on a maths class. Most humanities teachers I know have been asked to teach Y7 English at some point.
These classes from the teacher's first year will have different lessons from the completely different group of years and sets they will have in the their second year of teaching. And probably their third year. After about 5 years you will have a lot of lessons that you can recycle but you still need to plan for the progress of each individual class. Ability ranges vary from year to year. A GCSE class that only has kids aiming at A or A star equivalents will be different from a top set that has kids whose work goes down as far as a D for example. So you have change how you approach topics.
Behaviour and personalities also impact how you can teach a topic. Some classes might be well behaved and can manage group work or lots of discussion. Others might have challenging behaviour that would mean you have to really keep a lid on how often you let them interact with each other (or you!).
I had two well behaved Y9 top sets one year with similar ability ranges (according to scores). I thought great I can repeat lots of things across the groups. But no: one class was really chatty (in a good way), great at debates, happy to stand up and do talks, always wanting to read their work out, or read aloud. Many of them had handwritting that was terrible and they were really sloppy about handing work in and they did the bare minimum on paper. The other was the quietest class I have ever taught. If you spoke to any of them they looked at their desks, went red and mumbled, they hated anything where they had to speak to each other or me. Their written work was out of this world, way ahead. They wrote loads - the marking work load was huge! I had to teach these classes completely differently. this makes teaching interesting but damn hard work.
And finally probably during that 5 years where you have built a bank of lessons you will either change school (different exam boards, different expectations) or the government will revise the curriculum and everything you've taught has to change significantly.
So basically your planning changes fairly significantly all the time. I quit after a few years. I watched OH wreck his health doing 20+ years.