how do you know that it acts like it is non-existent?

to know you got the dlc, you can check game files on file explorer and see if you got the blood and wine , hearts and stone etc.

or in game menu you have the options to turn the dlc like outfits on or gwent on.

You have to reach a certain milestone in your game in order to start those DLCs. They aren't active from the start although they are in the game. Blood and Wine starts when you save a certain person during main quest, however playing it requires you being level 30+. Hearts of Stone is activated when you approach a certain noticeboard and read its content, again however, it's designed for high level characters.

Yes dls for the witcher.
Its better Dls in all game

If you're talking about the freebie alternative looks, etc. - you need to enable those from the game menu BEFORE you actually jump into your current gameplay again.

Story DLCs, as others have said above, only become available once you hit a certain level.

Does the main menu give you the options to play the DLCs without playing the main game? If it does, that should reassure you that they are, indeed, there.