Default bases can be set for individual charms from the client side by either setting the default-base for the model config of specifying the base when downloading by using the –base flag.
I.e
juju config-model default-base=ubuntu@24.04
or
juju download --channel 1.23/stable --base ubuntu@24.04
If neither of these are set then charmhub will default to the latest LTS version that is available for the charm on the channel being downloaded.