There are 5 hobby stories in The Sims Mobile. Each of them can be started using a special equipment. However, the piano hobby can only be unlocked by buying a special package.
The "Fancy Fingers" package includes a piano and a reflector. It can be found in the highlighted section. It costs 350 Sim cash. It unlocks the basic piano, so your Sim can start a hobby. You will gain other elements of this hobby by developing your history.
The piano hobby unlocks 5 items for the house and 2 new pianos. Like all other hobby stories, The Sims Mobile, this hobby has 10 levels of advancement.
Remember that only one Sim of the current generation can develop this hobby.