My DD2 is about to enter y10, and is in my opinion a borderline foundation/higher paper ability.
There is no point entering the higher tier for maths if you are borderline to pass, as faced with a paper which is more or less impossible you are more likely to feel down-heartened and fail.
Maths builds on itself, and lower sets naturally go slower. For the bottom sets it will be obvious they will struggle to pass so they won't be taught anything above grade 5.
The top sets obviously will be taught more or less everything.
There is likely to be one or more 'cross over' set where the final decision on which tier will be made on an individual basis quite late, possibly only in Feb y11. This will be based on what the pupil can do, but also on things like whether they make silly mistakes (an issue for foundation paper where a higher % is needed to pass) or get dispirited (an issue with higher paper where you only need 20-25% to pass but loads of the paper is inaccessible). The crossover sets may well be taught some grade 6 stuff too, but not more.
Science has foundation and higher, and combined/dual & triple.
Triple is normally only done by more able scientists as it gets quite hard and is a lot of science. Our school has 14hours of science per fortnight if doing triple and 10 if doing combined.
Triple you get 3 GCSEs, one in each science.
Combined/dual comprises all 3 sciences but you end up with a dual award so 5-5, 5-5, 6-5. With combined all your papers have to be either higher or foundation. You used to be able to mix and match but you can't anymore. There is still a lot of content! The content doesn't seem to be much different between the tiers, but the questions are I think simpler for foundation. The required maths skills are also lower for foundation. Because the content seems to be more or less the same the decision can be made quite late on, though again for some sets it will be obvious.
On the whole you follow the guidance from the school. They are the most experienced at guiding children to the correct tiers. (I will have to fight within myself on this, as I have a maths degree but DD2 struggles. I must be careful not to push for her to do higher if she isn't capable).
English is no longer tiered.
What you have to remember is that schools want pupils to get the best grades they can, and they also want borderline students to pass. They will be doing their best to get the best outcome for your child.