Apparently for Google it's done by aliasing www.google.* to forcesafesearch.google.com [1], or for DDG: safe.duckduckgo.com [2]
[1] https://www.draytek.co.uk/support/guides/kb-dns-safesearch [2] https://duckduckgo.com/duckduckgo-help-pages/features/safe-s...