All trading robots can be divided into several categories (there is no exact classification) - these are indicatorless systems, advisers using indicators, arbitrage trading robots, auxiliary or semi-automatic trading systems.
Indicatorless Expert Advisors usually use either martingale or modified martingale, in general, a game. This type of system is repelled by the fact that forex is a random variable and its forecast is impossible, therefore all moves differ only in the direction and rate, which increases with each loss. For example, advisers on averaging are very promising.
Conventional advisors using indicators. In each Expert Advisor, the decision to open a position is made when 1 or more indicators match (more about indicators), indicators can be different or the same indicator is viewed with different periods. This class is the most extensive. Very often found in the combined version with non-dictated systems.
Arbitration strategies are very difficult to describe and use. Now I will only say that this type of strategy plays on the difference in the course between different DCs on the same instrument. For example, EURUSD for one DC can cost very different for another and you can make good money on it.