The inventory is located in the menu that appears when you click on/touch the top of the screen.

In the PC version, right-click on an item to look at it and left-click on it to pick it up. In the mobile version, touch any tagged item to get two icons, hand and magnifying glass. Touch the magnifying glass to look at the thing and the hand to use it.

If you can use an item in the scene, when you left-click on an item in the scene, you'll get an option to use an inventory item on it, like "Use watering can on flowerpot." If you don't see such a line, either you can't use that item like that or there's another way to get the two things together.

To combine two items, left-click/use the hand icon on the first item, then left-click/touch the second item. If they can be combined, that'll do it.