Not sure about apps, I don't use a smartphone, but for my PC I have Brave as my internet browser and have set Yandex as my default search engine. For Brave you go to Settings>Search Engine>Manage Search Engines. Next you Add a search engine. This is what I entered:

Search Engine: Yandex

Keyword: :y

Query URL: https://yandex.com/search/?text=%s

Then click Add and it should show up on the list of Default Search Engines just above. Now you can make Yandex your default search engine.

Not sure about apps, I don't use a smartphone, but for my PC I have Brave as my internet browser and have set Yandex as my default search engine. For Brave you go to Settings>Search Engine>Manage Search Engines. Next you Add a search engine. This is what I entered:

Search Engine: Yandex Keyword: :y Query URL: https://yandex.com/search/?text=%s

Then click Add and it should show up on the list of Default Search Engines just above. Now you can make Yandex your default search engine.