Is it permissible for a Muslim to work in the field of programming mobile applications and selling them or adding advertisements to them to earn a living, and is it permissible for him to work in the fields of computer science, the internet, and technology companies?

The basic principle for working in the aforementioned fields is permissibility. Nothing is forbidden from it except what involves something unlawful or aids in it. Accordingly, there is no objection to taking them as a source of livelihood, provided that the applications are free of unlawful elements and that programming services are not provided to those who are highly likely to use them for unlawful purposes.