Am I just an idiot? I'm finding it impossible to figure out how to use roll20 as a DM

Maybe I'm just an idiot, but I simply cannot figure out how to use this as a DM. I am delving into virtual gaming for my group after spending 35+ years playing D&D in person at a table. I'm trying to create our current campaign within this virtual arena and am finding the interface of roll20 completely non-intuitive. Am I the only one that struggles with this? So, as a DM I have to launch the game in order to really do anything, right? Like uploading PDFs of maps I've made, etc. Ok, I get that. And I sort of figured out how to draw on them and add some limited tokens. What's stopping my players from logging in and joining the game when I launch it to do some edits? Why isn't there an "edit the game" function specifically for DMs? I purchased the monster bundle, because I thought it would cool to be able to include tokens and graphics of all the monsters in the MM, Volo's, etc. But when I go into the game to add a monster onto a map that I've created, and I search for a specific monster, none of the content I've purchased shows up. When I go to the game settings page and click on "add content" it only allows me to add the Volo's tokens pack, but there is no option to add anything from the monster manual or Mordenkainen's. Where do I find this? How can I access it in my game? I also signed up for a premium subscription so that I could add this content for all my players to access, but it won't let me add it. And nothing shows up from the monster bundle I purchased when I search for tokens of monsters. This is exceptionally frustrating, considering I already spent money on the hard copies and had to repurchase the digital roll20 versions. But I get that. Still, you'd think they'd make the interface make even the slightest bit of sense to figure out, at least to access the content that you freakin' just paid for on the site! Seriously, maybe I'm just a moron and an old Gen Xer that can't be taught new tricks. Not sure what I'm asking for here besides validation that maybe I'm not stupid? Ugh...

Nothing stops your players from launching the game themselves at any time (whether you're in it, or not). However, what they can do is severely limited. They can only see the map that currently has the Player banner on it. They can only see character sheets and handouts that have been set to that player or All Players, they can only see folders if they contain something they have been granted access to. Long and short of it, they can't mess with anything you're doing, and they can only see things in the chat window. As for your paid content, I haven't bought anything, so not sure how that works. You may have to go to your game page (not launch the game) and check your settings to see if the purchased content is loaded for that game and if it's set to be shared with your players. But no, you're not stupid. There's a learning curve with Roll20.

The purchased Compendium sources are determined on your game settings page. You need to choose the Compendium you will use (in your case D&D 5e). However, if you have chosen to use the official sheet, this will be set by default. The "Add content" feature you are talking about sounds like the "Addon" feature on your game's home page, that allows you to add certain types of mostly non-art purchases: small adventures, card decks and the like. If you have purchased the Monster Manual, Volo's etc, you do not need to add anything. Make sure the game you have created has a sheet chosen (I would suggest D&D 5th Edition by Roll20 for most D&D 5e users). When you log into the game, all of this should be waiting in the Compendium tab of your game, ready to drag and drop onto the tabletop. It's not your age. You may be a Gen Xer, but I'm a Baby Boomer. It's just coming to a new thing and learning how it works. You will get it. My suggestion is to check out the  Roll20 Crash Course  on Youtube, or my own video,  Starting from Scratch . Roll20 has started a brand new intro video series (and good for them, most of them are very old), but I haven't watched it yet.

If you don't want the players to see what you are working on you can move them to a different "Page!" 

No Paul it's definitely not you. I've been learning Roll20 for the last few weeks and I was surprised at how non-intuitive everything has been. It's not as polished as I expected, but I'm slowly coming around to the idea that it's more of a "framework"? for running any kind of TTRPG rather than a dedicated product for running DnD 5E.

It is definitely a learning curve but after spending a day messing with it and googling "how to's" I was able to pick it up fairly quickly. Not sure about compendium sources as I haven't bought anything and added it. But take your time and it'll come. I'm still figuring things out even after 5 weeks of using it now lol. A good idea is to make a free "test" account and log on from a different PC or laptop. That way you can build through your DM account and test how things work from the player perspective as well. 

Compared to the other VTT, Roll20 is a piece of cake.  You tube is your friend when you’re new.

Some mentioned it before, but wanted to add to it: You can make a page for the purpose of a landing page for the players if they enter a game between sessions. I have a handdrawn picture (thanks to a player of mine) of all our characters and a headline for the current chapter on it. I know some other dms use it for memos or a map of the current location. Just remember to put the playersbadge there after every session. Or you could try the communication thing where you tell them not to start the game at your usual prep-time. Some might say thats a radical approach, but it worked for me every time I used it. ;) About PDFs: There is no possibility to upload PDFs yourself. You can use books which you can purchase through the shop, but that ain't PDFs either. Those are highly customized versions of the books, specific for use with Roll20. I never used those, so I can't help on how to use them. Formats for maps (I know of) are jpeg (recommended, because small filesize) or png. Animated things are also possible, but I'm not familiar with those. If you have more specific questions, feel free to ask. I found this community is really helpful and try my best to give advice where I can myself. :)

If you attempt to upload a pdf, you will be presented with the option to convert a page of it to an image. You cannot upload it in pdf format.