Kernel configuration (`.settings.php`)

Baseline: main 23.0+ · Verified: main 26.800.0

Task	Read
Sections: `connections`, `exception_handling`, `cookies`, `crypto`, `ui`, …	`rules/sections.md`
Mail via SMTP (`smtp`, per-sender SMTP)	`rules/smtp.md`

Which file is read

Main: /local/.settings.php if present (Since main 24.100), else /bitrix/.settings.php — only one. The /local file replaces the kernel file: copy every section, connections included.
Extra: /local/.settings_extra.php, else /bitrix/.settings_extra.php — only one; loaded after the main file, each top-level section replaces the whole section (no deep merge).
Put project sections (closures, secrets, routing, loggers, messenger) into /local/.settings_extra.php; it shadows /bitrix/.settings_extra.php, so move that file's sections over.
Kernel/admin writes (Configuration::setValue(), settings forms) var_export main-file data into the main file; closures there break on the next write. The extra file is never rewritten.
Site checker and services/quickway.php read only /bitrix/.settings.php — keep it valid. IsConfigFile() denies file-manager access to /bitrix/.settings.php only, not to /local/.settings*.php.
php_interface/dbconn.php (/local Since main 24.100, else /bitrix) is still required.