When using whole/sliced fruit, I primary in a bucket fermenter. You can put your fruit in either primary or secondary; or both. Primary is easiest, but you'll lose the most amount of fruit flavour & aromatics that way. Fruit in the primary will give a more "wine-ish" sort of flavour profile to the fruit.
Adding fruit to secondary will give you more fruit flavour & aromatics, but if you secondary in a carbouy like I do, you can't contain the fruit in a bag & you have to be able to fit the fruit through the carbouy neck. It eventually ends up as sludge on the bottom.
Adding fruit to both will give you more depth of flavour, but makes things a bit more involved, though still relatively easy to do. I suppose you COULD secondary in a bucket fermenter, but I don't due to headspace/surface area & possible oxidation issues.
All depends on how much strawberry flavour you want in your cider. More berries = more flavour. The longer you leave the fruit in (to an extent) the more flavour will be extracted. Either way, I'd put those berries through a freeze/thaw, freeze/thaw cycle. It will help to soften it up & break down cell walls so more juice/flavour can be extracted. You could also use a dose of pectic enzyme.
Hope this info helps. Regards, GF.