The angels and fallen angels exist in a dimension parallel to our dimension. Both dimensions coexist without colliding with each other here on earth. On their side of the dimension, there are various cities of fallen angels on land, sea, and in the skies on Earth. They have their own ecosystem with water, plants, land and etc. Where we may see an empty lot, angelic structures and cities may exist, bustling with both evil and holy angels commuting back and forth in that location. So, where there is a wall on their side, there is nothing on our side obstructing our way. Our worlds do not physically intersect.
  We can walk through their invisible walls, and vice versa. Thus, we are mutually spirits to each other; they cannot physically touch us, and we cannot physically touch them while existing in separate realms.

Due to the fallen angels share power and their brutal violence, Heaven has established a dimensional border policy known as 'the hedge law.' This policy safeguards young worlds from the destructive force of fallen angels by prohibiting them from occupying the same physical location as young creatures. Angels possess the power to eliminate millions of humans in a single day. All of Heaven has witnessed the vicious brutality of evil angels in Heaven, witnessing the apocalyptic destruction they wrought upon Heaven's cities. Therefore, it would not be a fair fight to introduce these powerful beings among young civilizations, as it would undoubtedly interfere with their natural growth.

However, angels are legally permitted to employ technology that grants them the ability to observe our dimension. Their technology allows them to transmit holograms and Soundwaves in certain scenarios to communicate with us. This grants fallen angels the power to manifest as ghosts and various apparitions, as seen in seances, for example.

Nevertheless, technology capable of creating portals to enter our dimension is strictly prohibited by the Holy Angels, who oversee and police that realm. This is why evil angels do not physically appear in large numbers to unleash mass destruction as they wish. If one creates a loophole and make it pass the dimenional border, angels from will discreetly arrest the evil angel and take them back to their dimension.
However fallen angels maybe able to enter into our dimension if we build a portal for them to come through.  That would be legally allowed. It's considered implied consent. If humanity arrives to a place where they can build tech to open portals for angels,  to the courts of heaven that qualifies humanity to be seen as mature enough to legally consent to fallen angels coming to  their dimension.
There is a constant measuring of the maturity of individuals and people to determine how much legal interaction angels can have with humanity. This is done to provide a fair standard to judge both holy and evil angels. Even though this is spiritual warfare, everyone must fight fair. With this dimenional border policy the playing field is even so that there is enough freedom to choose good vs evil.