Spells are the exception
The Ready action states:
Sometimes you want to get the jump on a foe or wait for a particular circumstance before you act. To do so, you can take the Ready action on your turn, which lets you act using your reaction before the start of your next turn.
First, you decide what perceivable circumstance will trigger your reaction. Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it. Examples include "If the cultist steps on the trapdoor, I'll pull the lever that opens it," and "If the goblin steps next to me, I move away."
When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger. Remember that you can take only one reaction per round.
None of the above alters how/when resourses are consumed, so resources are consumed as normal, that is, when an action that consumes them is perfomed.
For example: if you ready an action to use Turn Undead or Attack with your bow, then the Channel Divinity or arrow respectively are only consumed when you react to the trigger, where as if you don't react they are not consumed.
Spells behave differently, because they are specifically called out in the next paragraph of the Ready action:
When you ready a spell, you cast it as normal but hold its energy, which you release with your reaction when the trigger occurs. To be readied, a spell must have a casting time of 1 action, and holding onto the spell's magic requires concentration. If your concentration is broken, the spell dissipates without taking effect. For example, if you are concentrating on the web spell and ready magic missile, your web spell ends, and if you take damage before you release magic missile with your reaction, your concentration might be broken.
Because you cast the spell as normal, you must also consume a spellslot to do so. Hence, the spellslot is consumed when you Ready a spell, regardless of whether you choose to react to the trigger.
