Behavior of Boost and Buck Converters
do they try to hold the output voltage constant

Yes they do, these converters are mostly used for supplying a circuit with a constant supply voltage. Although it would be possible to make a converter where \$V_{out} = n *Vin\$ there is not much use for such a circuit. Most circuits rely on/need a constant supply voltage.

Ideally you want the output voltage to be independent of changes in the input voltage.

Higher input voltage:

No problem for a buck/down converter as long as it is within the limits it can handle

For most boost converters, depending on how they're made, the output voltage would rise with the input voltage. So for a boost converter, increasing the input voltage too much is not desirable.

Too low input voltage:

A buck/down converter will output a voltage as high as it can, this will then often be the input voltage or a bit lower.

For a boost converter it will maintain the proper output voltage as long as it is capable.