Hello! What is the difference between a downloadable plain firmware file and a USB TTL uploaded one? Do they both contain the same codes? Is the USB TTL update more in-depth? Does it reprogram the whole system? Or do they both contain the same codes?
Do you mean transferring it via USB ttl, or instructing the device via USB ttl?
As Frollic indicated , we need a bit more info.
There are some devices that can load the boot loader via serial (kirkwoods etc) so you could consider that a more ‘complete’ flash - really not something you want to do if you can avoid it though!
Serial / usb ttl is usually used to interact with the boot loader for flashing via tftp. The ‘code’ being flashed is the same (usually, again rare unique devices do exist).
What are you trying to flash?
... and how.
Let's say I'm going to buy a security camera in a shopping mall (for example: TESCO/AUCHAN) that has a firmware version modified by hackers (USB TTL copied the code and modified). Then I switch on the camera, the camera downloads an central update. Will the update overwrite the whole system? Or would it definitely be advisable to update via USB TTL, obtained from the manufacturer?
In summary: Is it enough to just run the first update? Does it overwrite the whole system? Or is it advisable to reinstall it in some way? Is it usually done using a recovery file or USB TTL?
Is it enough to just run the update or does it not overwrite the hackers' code?
Sorry, which model of security camera is supported by openwrt?
None of them. It just works on the same principle.
So to be clear about it, you're not talking about OpenWrt at all, but some vendor firmware (if they used OpenWrt parts to speed up the development of their proprietary firmware doesn't change this fact) - apart from this not being a topic for OpenWrt support, everything is possible, there is no generic answer.
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Yes, I understand that. But I asked the question here because the manufacturer doesn't have a forum page of this kind. The users registered here understand firmware updates. That's why I asked on this forum.
That's not how it works. There is no rule or law that says that all firmware-updates have to work the same. OpenWrt does things one way, a device manufacturer may do things in an entirely different way, the hacked firmware you're using may do things again in a different way and so on.
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If you rephrase the question something like "can an embedded device have the boot loader rewritten" the answer is usually yes.
How that is done is going to be device specific. JTAG, an eeprom programmer, dedicated hardware in the SOC etc etc.
Usually any modified firmware will no longer automatically install any updates from the manufacturer anymore at all, because that would usually overwrite the modified firmware, making it pointless in the first place to install the modified firmware. What you'd usually do with modified firmware is...wait until a newer version of the modified firmware is available and then install that.
Also, USB TTL, ie. serial, is usually used to talk to the bootloader, not for transferring files in any direction. One would usually tell the bootloader to download and install any firmware file from a TFTP-server or e.g. a USB-device.
But this is all theoretical because you're talking about some non-OpenWrt firmware and we cannot know how they do things. You should go and ask them, not us.
Thank you very much for the answers! These are exactly what I was looking for. 
"would usually overwrite the modified firmware"
- Even if someone have modified the boot loader with an eeprom programmer?
I will e-mail contact the manufacturer and then ask for the device original firmware version.
"dedicated hardware in the SOC etc etc."
- I have never heard of such a thing. What does it mean exactly? Does it have an extra component that overrides the function of the device?
The bootloader and the operating-system running on the device are two different things. You usually do not modify the bootloader, it's the OS that is modified.
I read a long time ago that there are solutions to prevent the camera from recording what is actually happening. Could this be a hardware trick? If a hardware modification is made can it be overwritten with a factory or in the case of a router with an Openwrt custom ROM?
If I understand correctly, everything is related to the operating system, if it is modified to the original, there is no hardware trickery?
I have no idea what you're talking about, you're being way too vague.
See above. Also, I don't know your camera.
Some SOCs allow direct serial booting via the SOC core, eg Kirkwoods (my personal favourite devices)
This is similar to jtag access but needs no additional hardware.