The year 1 at my school (and other that I worked at) have two “golden time” sessions a week (I.e free play) - I think one was the whole afternoon and one was about 40 minutes. Obviously this can vary by school.
I think the big difference is the amount of time working.
In reception, for example in my class, once the children have completed the work, they have the freedom to play. Or they will be playing and called over on groups to work. Or sometimes, all the lesson will be on the carpet, so maths may purely be practical work on the carpet as a class or literacy is just on the carpet too writing on whiteboards for example. We do a gradual build up (normally) in the summer term - so start to give extra challenges and extensions when they’ve finished their work instead of going to play, sending them to their table to do the practical work and going around the desks to monitor etc. This year that plan hasn’t been able to happen much.
The big jump will be the reduced free play and more independent working. Your DC will have to sit at his table (normally these are still bundled into group tables) - and her place may be assigned instead of choosing each time (depending on how school does it). Year 1 will be expected to start working individually, not when the teacher/TA is at their table. They won’t just be able to finish and go and play all the time. It’s more formal - but not entirely.
Like an above poster said, year one still have role play corners, comfy book corners, toys. Lots of PE and music and Art and technology. Often very cross curricular at this age. He won’t be glued to his chair the whole time but he will be sat down more than last year. Special jobs often start in Y1 - the privilege of running something to the school office, Playground buddy jobs, our school does school council from Y1 up - the feeling of power!
The school should, particularly in light of the lost time this year, ease them into changes gradually, and take into account her SEN.
Don’t worry though, year 1 are still little - even I forget sometimes they’re only 5 and 6! They won’t be expected to be fully independent sat still all day
Year one is a bit of a jump but I’m sure your DD will be ok - I emphasised to my reception class the sense of “being bigger” in terms such as “look how amazing you can be by helping the new reception class who don’t know what to do”.
Best of luck to her - enjoy the summer break!