If certain aliens can enter the solar system at will : from how far could they be monitoring it ?

Same as we could hypothetically monitor "them", in other words a few light years. Maybe fifty or so light years at best.

Why would they want to "enter the solar system", assuming the improbable scenario that they exist?

I think they need very specialized probes to monitor us.

Ones that can convert information from our world into the quantum level of their reality. Then it can transverse limitless distances through many dimensions we don't even know exist yet. All of the extraterrestrial life that I've seen defies the laws physics in speed, energy, and flight characteristics.

To even think about them being associated with physical distance, as we perceive distance, is just silly. They are far outside that box friend.

The Kuiper belt or Oort cloud would be my guess. If there was a telescope on Ceres, our telescopes wouldn't even be good enough to detect it. Ceres is really close. If aliens aren't there, it would mean complex life is very rare. Part of being intelligent is having a desire to explore. Looking at lifeless rocks, or even planets with microbes wouldn't be very exciting, and would also be the majority of worlds discovered. A lot of the moons in our solar system look the same. Mars is actually more interesting than other rocks. Nevertheless, planets like Earth would be the whole reason for exploring in the first place. The only reason not to make their presence known is because their lives are too precious to risk getting killed by humans. The idea of a "prime directive" is cold war era thinking. If humans were to discover life forms that explore new star systems, it would inspire them to do the same. First contact would be positive, at the species level. There will always be discontent people. They shouldn't define humanity. Not when there's such a boon to humanity as discovering that interstellar travel is feasible. Humans have nothing to lose by being contacted by aliens. In contrast, aliens have a lot to lose. They don't know what kind of reception they might get. The risk of being killed is too great. In any case, the James Webb telescope completed all of its tests in February. It's ready for launch. If the JWST discovers oxygen-rich worlds all over the place, it would strongly indicate that intelligent life is common. There is more evidence that intelligence is an evolutionary arms race than there was evidence for evolution in 1920, when the Scopes trial made it mandatory to teach evolution in public schools. Therefore, abundant oxygen (20% of the atmosphere) found in several worlds, would likely mean the galaxy is teaming with intelligent life. Oumuamua may have been the first evidence of aliens surveying the solar system. Oumuamua was seen accelerating when it was far away from the Sun, headed away from us. Oumuamua was 10x as wide as it was long. No known asteroid has that shape.

The possibiliteis are only as vast as the type of technology they have.