Iphone

Go to Settings > General > Keyboard > Hardware Keyboard, then do any of the following:

Turn Auto-Capitalization on or off:

When this option is selected, an app supporting this feature capitalizes proper nouns and the first words in sentences as you type.

Samsung

To turn off auto capitalization on a Samsung device, you can do the following:

1. Go to Samsung Keyboard settings

2. Select More typing options 

3. Auto capitalization automatically capitalizes the first letter of each sentence. 

Auto-correct tools on smartphones learn how you type, and will automatically capitalize words if they detect the right context. For example, if you capitalize a word often enough, it will assume it's a proper noun.