What device am I supposed to use my TOTP authentication app on??

So I have a iPhone and MacBook that I both use daily. I’ve been hearing you need to keep your authentication app separate from the devices you want to use the 2FA codes for.
Which is both my iPhone and Mac
So what am I supposed to do then??

I have a Fitbit I never use and a old Chromebook but this just makes things inconvenient as I never use them anymore.
Having the app on my phone seems more …

2fa 2fa codes app authentication authentication app daily device devices hearing iphone mac macbook privacy totp