When it comes to cloud storage, you have two approaches. Encrypt your files before uploading, or use a privacy, perhaps self-hosted, cloud.
If you decide to use one of the big ones or maybe because you have an Apple subscription and will run with iCloud, then use Cryptomator to encrypt your files. This works on iOS, Android, and desktop alike.
With Cryptomator, the key to your data is in your hands. Cryptomator encrypts your data quickly and easily, afterwards you can upload them (protected) to your favorite cloud service.
For Android, you can also use DroidFS
Store & access your files securely. DroidFS is an alternative way to use encrypted overlay file-systems on Android that uses its own internal file explorer instead of mounting virtual volumes. This allows you to store files in encrypted, virtual volumes that other apps can't decrypt.
Currently, DroidFS supports only gocryptfs. This algorithm protects file contents and file names, but doesn't hide directory structure or file sizes.
Have a look also to our File encryption entry
A better option is to use none of the big cloud storage companies, but one of these options:
Nextcloud is a fully open-source storage and productivity platform that keeps you in control. It's a monster when it comes to features and you can seriously look to making it your full-blown android/google and desktop as well as iCloud replacement!
Zero knowledge end-to-end encrypted cloud storage, redefined. The first 10GB are free.
Seafile is an open-source file sync & share solution designed for high reliability, performance, and productivity. Sync, share and collaborate across devices and teams. Build your team's knowledge base with Seafile's built-in Wiki feature.