Hello!

Ever since Minecraft 1.14 came out, the lectern has always intrigued me. It is basically a way to let players choose one of 15 redstone outputs, something we've only been able to even come close to with 8 with item frames!
So, one of the first things that came to my mind was using a lectern to select potions to brew with redstone. Well, I got to work, and though it was a bit of a hassle trying to get 15 different potions into the system when the redstone outputs are side-by-side, I eventually made it. This is probably far from the most compact you can make it. but I just wanted to get the concept out there, so perhaps others could build upon it and compact it.

Anyways, enough with the backstory, how do you use it? Well, I have included two structure block files, one with the machine with none of the ingredients inside, and one filled to the brim with ingredients. In the one without ingredients, you need to place the ingredients in the droppers/chests in the back (don't worry, they're marked with item frames, just remember to load the structure with entities on. Once that's done (the indicator lights should be on, but the Blaze Powder light requires 15 stacks of it, and the water bottle one 15 water bottles, so don't worry if the light is off for those, as long as you see them in the brewing stand, you're fine), flip through the "Select Potion Effect" lectern to find the potion you want, making sure you have the necessary ingredients for it. Then, go to the "Select Modifiers" lectern and select what modifiers you want to apply to the potion. Then click the brew potion button and wait for the potions to finish brewing in the stand. Viola! 3 Potions of your choice!
EDIT: In hindsight, I forgot that the modifier lectern neglects to mention that you need both Dragon Breath AND Gunpowder for a lingering potion.

To install these structures into your world,
1. Download the zip file provided
2. Navigate to your minecraft world folder in the "saves" folder in your ".minecraft" folder ((Your Computer Account)/AppData/Roaming/.minecraft)
3. Extract the "generated" folder into your world's folder
4.Open up the world (with cheats enabled) and type /give @p structure_block
5. Place it and put it in "Load" mode
6.Type either "brewing" (empty with ingredients) or "fullbrewing" (filled with ingredients) in the Structure Name box
7.Load the structure! (be careful not to have anything important inside the white outlined box)

I hope you find this useful. Thank you!