You’re not necessarily ignorant, just inexperienced. Nothing wrong with that. Everyone has to start somewhere.
And you are correct to ask because even the Internet can’t possibly have every available property for rent.
And as far as real estate agents are concerned, they get their fees reimbursed by the owner of the property once a tenant is found so there is no cost to you (or at least I’ve never heard of such a thing )
A combination of Craigslist plus real estate agents should get you most of what’s available in any given area. At least in this day and age those are the best options.
There are still places which are only available through word of mouth if the owner can afford to be choosy and be without rental income for awhile.
Over 20 yrs. ago when I first moved to Philly to take a teaching job, one of the other teachers there introduced me to the owner of the previous apt. in which she had live .
It was not advertised anywhere because it was on the second floor above a Doctor’s office. Because the two floors were not sealed off from each other they had to be quite choosy about finding someone trustworthy to live there. Just putting up a general ad would not be workable.
And obviously, as a doctor they could afford to be without rental income until an acceptable tenant was found.
But, anyhow, most widely available places nowadays will show up online or one of the several other alternatives people have listed.