I've been reading through various D&D 5e questions on rpg.se and this question (Does Sheathing and Unsheathing Separate Weapons Count As a Full Action?) has brought to my attention the free item interaction rule (from linked question's accepted answer/Basic Rules p. 70):
You can also interact with one object or feature of the environment for free, during either your move or your action. For example, you could open a door during your move as you stride toward a foe, or you could draw your weapon as part of the same action you use to attack.
If you want to interact with a second object, you need to use your action. Some magic items and other special objects always require an action to use, as stated in their descriptions.
Reading this and the accepted answer from that other question, I get that to switch weapons mid-battle means sheathing one at the end of one turn and drawing the other at the start of your next turn (I'm ignoring dual wielding in this question for simplicity).
I've always assumed that combat starts with weapons drawn, meaning you could use your free item interaction to draw your second weapon and attack with both (action and bonus action) on your first turn. It's perfectly reasonable to assume that the PCs have their weapons ready if they're expecting trouble.
But in a situation where the PCs are not expecting trouble, then if it's actually the case that you use that free item interaction to draw your primary weapon at the start of your first turn, then obviously that won't work (since that's two weapon interactions; this is all assuming you don't want to use an action, of course).
So, does combat start with at least one weapon drawn automatically when combat is not expected, or does drawing your weapon take up your first turn's free item interaction?
If the latter, this also raises the question that if an enemy wins initiative (or you're "Surprised" by them) then you can't do opportunity attacks since you don't have your weapon out yet (unless unarmed strike is an option), but I don't want that to get anyone side-tracked from my main question here, which is about the free item interaction thing.