Samsung
Auto-Punctuate on Double Space Tap auf Samsung
On Samsung phones, double-tapping the space bar can end a sentence with a period, but only if a specific keyboard option is active. This shortcut belongs to the Samsung Keyboard settings and works independently of autocorrect or predictive text.
By default, this option is disabled. That is why typing two spaces in a row on a fresh Samsung device just inserts a space and no period appears. If you want the shortcut, or if you keep getting periods you did not ask for, you can switch the behavior yourself.
To find the setting, open Tap on Settings ➔ Tap on General management ➔ Tap on Samsung Keyboard settings ➔ Tap on More typing options ➔ Enable or disable Double tap space bar to add period. Turning it on adds a period automatically whenever you tap the space bar twice at the end of a word. Turning it off restores the plain double space with no punctuation.
This option only affects the built-in Samsung Keyboard. If you use a third-party keyboard app, check its own settings for a similar shortcut instead.