On the path C:\ProgramData\Maniaplanet, are X gb of Trackmania files (for me 6 Gb). I wanted to move them on D drive, but how to make the game work after this? Symlink will make the game to think, that moved files on D drive, are still in old place on C drive.

1) Create anywhere on D drive a new folder, where you would want to move X gb of Trackmania files from C drive (or use already created folder). I created folder maniaplanet1 on the path D:\game\.
2) Ctrl+x, Ctrl-v - cut, paste files from C drive to new folder on D drive (from C:\ProgramData\Maniaplanet to, in my case, D:\game\maniaplanet1).
3) Delete folder Maniaplanet from C:\ProgramData\ , which after cut-paste operation, must be empty. This is need, because on the step 5, we will create an illusion-folder Maniplanet and it would be impossible, if on this path would already exist a real folder with same name.
4) Open Command Prompt window as a administrator. If you do it right then the on Command prompt, the first line would be C:\Windows\system32> , not C:\Users\your system name> .
5) We have our files on D drive, but the game want to see them on C drive, so, we would create and illusion-folder (folder with labels to D drive) there. Bassicaly we didn't change anything from the game perspective, just added 1 more step (folder) in their file search. For creating illusion-folder on C drive with link to the files on D drive we would use symlink. For this we need to write in Command prompt next line:

mklink /J C:\ProgramData\Maniaplanet "path for folder with files"

mklink - is the command for creating symlink
/J - represent the type of symlink which we want to create and it's mean directory junction.
C:\ProgramData\Maniaplanet - is the path for new illusion-folder on C drive, which will create the mklink command.
path for folder with files - is the path on D drive where we moved X gb of game files (D:\game\maniaplanet1).
In my case this line in the Command prompt was:

mklink /J C:\ProgramData\Maniaplanet D:\game\maniaplanet1

6) Press Enter button.

On the path C:\Users\your system name\Documents\Maniaplanet there are also several Gb of Trackmania files (3 Gb for me). This folder is familiar for gamers, almost all games use it as a save location, but 3 Gb (and this is the begining, + many Gb in the future) is also too much for me, i decided to free C drive from this folder too.

1) I created folder "maniaplanet doc" on the path D:\game\
2) Ctrl+x, Ctrl-v - cut, paste files from C:\Users\your system name\Documents\Maniaplanet to D:\game\maniaplanet doc
3) Deleted folder Maniaplanet from C:\Users\"your system name"\Documents\
4) Open Command Prompt window as a administrator
5) mklink /J C:\Users\your system name\Documents\Maniaplanet "D:\game\maniaplanet doc"

"D:\game\maniaplanet doc" - this time, path to folder with files, is in "" bracket because of one name in the path include space, without bracket mklink in this cause wouldn't work.
6) press enter.

That's all. Information of how to use symlink i taked from this site, which was on the first page of search.
https://www.howtogeek.com/16226/complete-guide-to-symbolic-links-symlinks-on-windows-or-linux/