In one of my stories there's a planet which is tidally locked to its red dwarf parent star, I imagine terrestrial life evolved on its twilight zone and then spread to its more extreme sides. Some plants found a way to live in the dark side of the planet using symbiotic chemotrophic bacteria as a power source, over time these plants colonized most of the dark side of the planet, even losing their photosynthetic abilities eventually, my question is how these plants would adapt to this part of the planet, in which shapes would they grow? would there even be different species?
What you have described is essentially a Fungus
Your organisms are essentially non-motile macroscopic consumers which lines up pretty nicely with the niche held by fungus and slime molds on our world. Like fungus, your dark plants will likely evolve to stick much closer to the ground since they don't need to grow upwards to get sunlight. They will also want to evolve to stay close to the ground to be more cold resistant (important considering where they live.)
This means that over time, they will transition to forms that are mostly roots and not much else making them probably look something like this:
We have this.
The Indian Pipe is a plant that is fed by fungi. The fungi are apparently parasitic on trees (at least, there is no known benefit, though I retain an unfounded skepticism whether the trees would retain an endomycorrhizal relationship to feed a fungus, that then retains a relationship to feed a plant, all with no quid pro quo at all!
You'll want a compelling explanation of the apparent free lunch that powers your plants, but having a light and a dark side to the planet could maroon large amounts of chemical energy for you to work with.
In your situation, you should also be aware of anti-solar cells. (gee, I wonder what Monotropa look like in infrared light...)
using symbiotic chemotrophic bacteria [...] even losing their photosynthetic abilities eventually
this would turn them into simple parasite. A symbiotic relation requires both side to do something, not to just get.
Since these organisms do not need to photosynthetize but just to suckle on the bacteria, they won't need leaves and probably not even roots, just receptacle where they host/trap the bacteria.
Though I suspect the dark side will be too cold to sustain any meaningful life, except maybe around some geothermal place.
If I am not wrong, this would push these organisms toward carnivory. Only so much can be extracted from soil, and as in situ nutrients are depleted this nichs becomes saturated.
The plants need not be blood thirsty animated venus flytraps. In the Eyeball world like this winds would be constant and relatively strong. Airborne nutrition could be filtered out. Think of a dry land reef system? Although prickly thorns could be a thing to peerce and hold any wayward animals that was blow it's way. Airborn plants could also be a very real thing... Heavy thorny tumbleweeds?