In iOS 12 and macOS Mojave, Apple made huge strides to the way you create secure accounts and fill passwords. This even goes for third-party apps on iOS.
On Mac, Safari can now automatically recommend a random, secure password when signing up for services. It can automatically fill this login the next time you visit the site (and it even handles multiple logins now, too). This gets saved in your Mac’s keychain and synced to other devices as long as iCloud Keychain is enabled in Settings and System Preferences.
Your iPad and iPhone can also recommend strong passwords in Safari but also in third-party apps. Some apps might not be compatible yet, but I’m really impressed at all the apps iOS can fill my passwords. Look for the Passwords… button in the keyboard suggestion bar.
Bonus Points: On iOS, you can even bring third-party password managers like 1Password into the mix. Go to Settings > Passwords & Accounts > AutoFill Passwords, and tap the one(s) you want.