I used OBS Studio to create five recordings.
Now I need to submit them as just one file on YouTube, so my five .mkv files has to be merged into just one file.
Can it be done in OBS Studio, if so how?
(I didn't manage to do find a way)
Are there any other free tool to download or use online that can fix this in an easy and controllable way?
(Preferable with the possibility of transition effects, like in this Premiere Pro demonstation if possible)
This is absolutely not a problem. You can do this with MKVToolNix (specifically with it's GUI, since that is what you have requested).
I am on Debian GNU/Linux, if you use something different, your screenshots will differ, do not worry about it.
First start the application:
Add the very first file to it:
Now you will need to add or drop the other files.
Doing so you will get a question of what to do with the files you've added:
Select "Append to an existing source file".
Now you only need to provide the name of the output file and press "Start multiplexing".
In case you will be unable to use MKVToolsNix for whatever reason, there is an older tool which is able to append files quite well: AVIDemux.
The process is trivial. Start the program and open your first file.
You then select append from menu and select files that you want to append.
That is it. Now you simply save the output. Take a note that the video and audio streams are set to copy.
