Test both.
And have the battery load tested.
The ignition switch and the bulkhead connectors also.
IMHO
Assuming the buzz sound was the starter relay attempting to make a contact but the battery voltage was too low that is one posable scenario
That it started the next day suggests the battery was not low
So what else...
If you had a bad connection to the battery or a bad ground that got worse with heat, that could cause the issue as described.
So check and clean the battery post connections, the ground wire/s. There should be one at minimum from the back of the pass side head / block to the firewall.
Also look for broken wire strands in the ground straps. A few strands might be enough to turn over a cold engine but not enough for a hot engine?
To test the ignition switch and associated wires and connections, get a remote starter switch. It is a low current switch that goes between battery positive and the terminal on the starter relay that the yellow wire goes to.
If the remote starter switch cranks the engine you know the relay and the solenoid are ok
Start looking at the ignition switch and related wires
To test the starter solenoid you can bridge between the battery post on the starter relay and the post on the starter relay the wire that goes to the solenoid is attached to. (I don't like method as it melts the posts and nuts) Swapping in a known good starter relay would be my first choice
If the solenoid test cranks the engine and the relay test did not, then the relay is the likely candidate
