Hey Mad,
It depends on how the game was developed whether or not it can be played singleplayer or not. Take a look at these 2 games. Look on the right side of the page under "Is this game relevant to you?". It will state there if it can be played "Single-player".
Blanc states it is an "Online Co-op", no "Singleplayer" listed. The game
Distrust: Polar Survival states it is a "Single-player" as well as "Online Co-op". The game "Distrust" also states it is both, singleplayer and co-op, in the description top right. In some cases it will list "Singleplayer" in the Tags list as well. Blanc can only be played On-line Co-op
Here is a
Walkthrough of "We Were Here" giving an idea how Online Co-op works