>When you go to the bathroom, and you fail to execute the "look at the waterflosser" thing, what do you do instead? Do you look somewhere else?

I look whichever direction I'm moving. Or I stare blankly while I think of something else. Or I look at the clock, which technically requires turning to look past the waterflosser but did not involve the flosser registering to my attention. Whatever was in line with what I was doing/thinking about before walking in.

>What happens when you put a sign on the door saying "remember the waterflosser?"

Never tried. No longer live there, but I could attempt something like it.

>What happens if the waterflosser is right next to whatever it is you most frequently need to do in the bathroom? Do you use the sink regularly?

It was and I did, and so I usually had my gaze wander to it while I was in the bathroom at some point. Probably 80% of the time. This usually meant that I'd remember to use it if I hadn't already that day.

>Do you brush your teeth? (If so, then it seems like somehow somewhere you got a habit of doing that - how did that happen?)

Yes. I have a Beeminder goal for it (this is my "anchor" goal that I need to update almost every day, which will ping me with a phone notification if I miss more than a couple days, or more often forget to enter my data for a couple days). Most mornings I remember before I leave the house. I also keep a travel brush in my backpack and another at work, for the mornings when I don't. (Somewhere between 1/3-1/4 of weekday mornings. Weekends it's more like 80%.) Evenings I almost always think of it while I'm winding down for the night, probably 90% of the time, unless there is a distraction like a visiting guest.
I think I had it as part of a routine when I was a kid, and definitely had an ironclad morning routine during my first internship, during high school, which included a shower and brushing my teeth. Before having this goal I think I remembered to brush my teeth about once every two days.

>What is "remembering to eat" like? (it seems like you must at least successfully do that most days)

You'd think! But no, not really. (If I rated the problems in my life right now by (tractability x impact) this would win.) I generally will not think about eating until my stomach hurts from hunger. Sometimes that passes and I forget about it before I have pulled myself away from whatever mildly interesting activity I'm doing. If that happens, and it doesn't reoccur, then I will probably not think of it again until I notice my movements and thoughts becoming sluggish from lack of energy. (On one occasion, now years ago and thankfully unrepeated, I continued this for about 48 hours over a Thanksgiving Weekend, consuming only water and half a bag of stale Chex Mix. It was a bad time.)
To mitigate this problem I try to keep a lot of extremely low-effort food around - yogurt drinks in the fridge, large cans of cashews on my bedside shelves, and when I noticed myself losing energy, eating some of those and then trying to get up to make something slightly more substantial like a couple PB&Js. Despite this accommodations and willingness to order takeout, I think I've averaged about 1.8 meals a day over the last two months of weekends, much of them taken as grazing just enough to keep me not collapsing.
Luckily food at Google comes free and pre-cooked into convenient forms that require only walking, no thinking. And I am doing enough attention-intensive activities that I notice much sooner that my function is impaired, so I intervene before I start losing the ability to move or act like a person.

I have no routines and have no idea how to gain any. All advice I've ever found is about modifying or extending routines.

Reflexes, I have some, but weak ones. I have never deliberately inculcated a reflex AFAICT. My attempts look like the waterflosser thing when they sorta work, and like nothing at all the rest of the time.