Why does the amplifier become a hysteresis comparator after adding positive feedback?

Don't be entangled in these superficial concepts. Comparators should be functionally defined and are essentially amplifiers.

Comparators and op amps can be regarded as one kind of things, but there are more limits. If the op amp is connected to a comparator, that is not a problem.

In principle, the principle is similar. Compensation is omitted inside the comparator. Circuits such as analog bias are comparators. The comparator requires speed, and the operational amplifier requires stability. If the requirements are not high, the two can be interchanged. Generally, the comparator is OC output.

Many times the op amp can be used as a comparator, but pay attention to the differential mode input voltage range. Some op amps have back-to-back clamp diodes inside the non-inverting and inverting terminals. This kind of op amp is likely to consume a lot of current when used as a comparator. .

ADI has an application note that does not recommend using an op amp as a comparator.