Word for a non-professional job done as a hobby

I think you are looking for amateur:

• One who engages in an art, science, study, or athletic activity as a pastime rather than as a profession.

(AHD)

or:

• you should probably define yourself a "programmer", the fact that you do it professionally or not is not related to the competence you have developed.

I think hobby itself would do just fine. Hobbyists work at their craft when it’s fun and convenient, but don’t make the necessary commitment needed to get to the professional level.

"Selling apps on Play Store is my hobby".

So you may be a "hobbyist programmer ". Some people actually hate that name.

a hobbyist programmer is somebody who spends 10 or more hours a month programming, but is not paid primarily to be a programmer.

---

What you're describing may also be a sideline career.

An activity done in addition to one’s main job, especially to earn extra income

[As modifier]: a sideline career as a stand-up comic

"He founded the fast-food company as a sideline to his petrol station"

Dilettante fits the bill nicely:

a ​person who is or ​seems to be ​interested in a ​subject, but whose ​understanding of it is not very ​deep or ​serious (Cambridge)

...especially if you are emphasising the non-professional aspect.

You might consider avocation, which means a hobby, from the Latin a(b) (away) + vocare (to call), i.e, something that calls one away from the profession or occupation at which one makes a living. There is no implication that you necessarily are a beginner or are unaccomplished at your avocation, just that it's a sideline.

You might consider, pastime

An activity that occupies one's spare time pleasantly: Sailing is her favorite pastime.

[Middle English passe tyme, translation of French passe temps : passer, to pass + temps, time.]

The American Heritage® Dictionary

A slightly dated expression, but one I still use is a labour of love.

If you're not amateur level (which would be correct world, implying that you do something just for the love of it, but also has a negative tone), you may use

craftsman - someone who is skilled at a craft, but doesn't necessarily has to have it as a (current) job.

-> Software Craftsmanship

Enthusiast - a person who is very interested in a particular activity or subject.

An “amateur”

An amateur is when you enjoy doing something (and are particularly good at it) as a hobby (for fun not money) instead of doing it professionally (get paid to do it).