Whether or not you get to choose which slots to play on for your free spins depends completely on the individual casino and offer.
At some online casinos with free spins, your bonus will only be available for one particular game. In that case, you won’t even be allowed to look at other titles until those free spins have been played. With other bonuses, you’ll have a selection of titles to pick from and use up your bonus spins.
With offers that give you some choice, you should take care to make sure that you’re getting the most value from your spins. They will have a set maximum price that they will play for each turn, but it’s not always a bet size that’s available on each game. If you pick a slot that doesn’t have a betting option for the maximum price per spin, you’ll play with the closest amount that’s lower than that maximum. That means you’ll lose some value in the process.