I’m rendering in cycles, I’ve added the glare through the Compositing nodes. Though, when I render the scene there’s no glare.
When you render or when you save as png and open the png?
You should have no problem rendering and seeing the glare in the render inside Blender, if you do not see it share a pic of your compositing setup.
Saving the glare with a transparent background is a different matter, due to png not being compatible with Blenders Alpha. You would have to save in exr and use a program that can read exr properly.
I’m rendering in TIFF. I’m still in the learning process, and I’ve seen a video of a guy explaining why rendering in PNG was bad so…
Unfortunately I do not think tiff is much better.
There are different methods (workarounds) for this.
This first video explains the problem a bit.
Unfortunately it is not a simple problem.
so basically the png format is the only one which works with bloom
Glare works in bright areas.
If there is a bright area in the scene, try adjusting the threshold value of the glare node by starting at zero and gradually increasing the value.
No PNG has the same problem, it is not the bloom, the problem is the transparency. You can export bloom with a non transparent background in any format.
The viewport has the glare wich is not present in the render
Do you mean seeing Composing as a rendering in Viewport?
This is not handled exactly. I understand that it is approximated or overrepresented. 
nah I’ve got it. i was using the view port as a reference, but i should have used the node background thingy instead. Is there a way to zoom out this image?