Well, legal manga apps usually have partnerships with publishers and follow copyright laws. However, there are a lot of shady apps out there that violate these laws and are definitely illegal.
In most cases, they aren't bad. But some poorly developed manga apps might have issues like battery drain, compatibility problems, or security vulnerabilities. So, it's important to choose well-known and reviewed apps.
There are a few. Tachiyomi and Crunchyroll Manga are worth checking out. Tachiyomi is known for its customization options, and Crunchyroll Manga offers simulpubs of ongoing series. It depends on your specific preferences though.