I noticed that previous default themes used to have /languages/ folder which is now gone.

1. Is it now a good practice to not include the /languages/ folder when creating a custom theme? How do theme users translate it without this folder?
2. Where should MO files be uploaded to not have them overwritten when a theme is updated? If they are located in /wp-content/themes/theme-name/languages/ then after each theme update they'll be overwritten. Should they now be uploaded somewhere else?
3. The default language is en_US. When I create en_US.mo file and use https://codex.wordpress.org/Function_Reference/load_theme_textdomain the strings are not updated. But if I change the name of the MO file to e.g. en_GB.mo and update Settings > Site Language to "English (UK)" the strings are replaced. How can I keep en_US and update the strings?