With regards to font scaling, for accessibility you'll want to generally respect the user's settings, for example, some people choose to use their devices with smaller or larger fonts by default.
So yes ideally you'll always want to scale it to user's preferences using rem units, rather than specific pixel sizes such as 16px.
The reason 16px always comes up in this conversation is that it's the default value of 1rem, where users haven't altered their preferences.
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