This is a classic (not to say retro) point-and-click game. On the PC, when you want to look at something, right-click on it; when you want to interact with it (pick it up, talk to it, use it), left-click on it. In the mobile version, tap on the item and pick the hand to interact or the magnifying glass to just look.
Hover at the top of the screen to see the menu, which contains your phone, your inventory, and the Options menu (where you can save and load games, see your achievements, and adjust some of the settings). You'll see that there are more settings available than in the previous games, though they're still pretty primitive.
In this game, you can switch between being Rosa and being Joey. To change from one to the other, either press the Tab key (PC only) or pull down the menu and click on R or J. Joey's two abilities, blowing on things and using his tie, are now in his inventory. To use them, drag the breath or the tie onto the item you want to use it on.
It should be noted that in the PC version, you can't play in a window. Pressing Ctrl-Esc during the game will drop you to the desktop, but the game will be chucked when you come back. If you need to check something on the desktop, it's better to save your game and close it, then restart and reload when you're done.