Very little is inherent to collaborative robots, and this is especially true for sensors.
The term 'collaborative robot' is still new in the industrial world and there is a bit of hype and variation in meaning in what exactly they are talking about.
In general a collaborative robot will be one that expected to do its work with a human in its workspace. There is ISO standardization around this and the focus is on safety.
In the domain of collaboration, there are two main things we want to do in my view.
1) We want to know where the human is relative to the robot so we can operate the robot in a more performance oriented manner when the human is far enough away. This could be as simple as always moving safely and stopping (moving even more safely) when in contact with a human. It could be as complex as tracking human kinematics and moving more 'dangerously' based on the distance between the robot and person and the dynamics between them. Think of the knife-between-fingers scene in Aliens.
2) We want to receive commands from the human so that the robot can collaborate. This might be lead-through teach and play programming, or one might simply sit down at a robot workbench and block off the area you want to use with your arms, like a little kid at the art table establishing their space.
You can see this is a big domain with a lot of possibilities. So when it comes to specific products, you will have to sit down with each vendor to find out what exactly is 'collaborative' about their robot and what package of options is needed to achieve that collaboration. There is no standard of what collaboration means, nor what sensing is used.
One thing to note is that collaboration is completely separate from the actual task of the robot. End tooling, tool changers, fixtures, etc all need to be considered in addition.