We can use in MIT App Inventor 2 these sensors:
Can we use another sensor e.g. IR Sensor
or Step Sensor
?
And if we can, how can we do it?
Which component should we use to access other sensors?
There is no IR component
available in MIT App Inventor, but you can create your own App Inventor Extension.
More information about how to create an extension see here, however that will be more advanced and will require some Java skills...
As an example, see my Barometer Sensor Extension.