A single-process Machine runs an entrypoint / cmd as defined in the app’s dockerfile or its fly.toml (docs).
One can choose to run a different entrypoint / cmd by specifying a fly m run --entrypoint <here> [--schedule daily] ... but I am not sure if it works for scheduled Machines; you could give that a shot and let us know if it does.
The usual way: Whatever script you want run every day should be the dockerfile / fly.toml entrypoint / cmd instead.
If you want cron-style jobs, however, there’s a couple of ways to do this (superchronic, crontab with apps v1 not machines).
In general, to understand what a Machine (apps v2) is, see if this answer helps: Advantages/Disadvantages of Machine Apps - #2 by fideloper-fly