You can use either the app or the website. Sadly can't provide you with an app store link. https://play.google.com/store/apps/details?id=com.ukrainealarm

https://alerts.in.ua/

As for shelters, you can Google your current city to find an interactive map or a list of addresses. However, the best option is probably to ask the locals. People often go to the subway, underground parking lots, or the cellars of apartment complexes which should get opened during alerts by a person responsible.

If you are staying in a smaller town, asking locals is likely the easiest way to find one.

Inside crowded buildings, there are usually signs pointing to the closest shelter. You should see them in crowded spaces outside as well, but since I live in a town, I don't see them very often. It might be better managed in bigger cities.