A few weeks ago, I bought a Samsung S9. The hardware is great, but the software is terrible. The first thing I did was uninstall the Samsung bloatware and install my favourite apps.
I replaced Samsung Clock with Google Clock. Google Clock allows you to set multiple timers, among other things. You might also want to use your own alarm clock app, such as the excellent Sleep As Android. After a lot of trial and error, I found out how to set the default app that Google Assistant will use to set alarms and timers.
Changing the default timer app
All you need to do is say “Okay Google, set a timer”. Do not specify the time. Google will ask you when you want to set the timer. Do not say anything.
There will be a bubble that lets you enter a timer duration. In the bottom left corner of the bubble, there should be a small clock icon. Click this icon to select which clock app to use. Select your favourite app, then set the timer and click confirm.
Google will now use that clock app by default.
Changing the default alarm app
The procedure for setting the default alarm is the same. Say “Okay Google, set an alarm”, then select the app to use on the time selection bubble.