There is no exact rulebook concerning demons, possession, or oppression.
Generally speaking, when a possession happens, it is because the person in some way invited for it to happen, such as by being involved in the occult or trying to communicate with spirits, either because of a reckless, immature curiosity, or for darker reasons.
But it can theoretically happen to anybody. Two Biblical examples are Job & Sarah, daughter of Tobit. Job experienced intense demonic oppression by Satan, and he was a righteous man. Sarah was being tormented by the demon prince Asmodeus, and she was also righteous.
There is nothing that says a demon absolutely can’t be banished except through an exorcism, but an exorcism would be the ordinary way. In very extraordinary scenarios, God might be permitting Satan to continue to torment somebody (i.e. St Padre Pio). St Padre Pio experienced fantastic visions, private revelations, and supernaturally imbued knowledge, and he was routinely tormented by messengers of Satan to keep him as humble as possible; spit on the face, thrown from the bed, beaten, flogged, serenaded by demons appearing as naked women, laughed at, insulted, etc. Satan would appear in the guise of penitents, friends, an angel of light, or the Blessed Mother. Letters from his spiritual adviser were blank until he applied holy water to them. The list goes on.
There are reports that possessions are on the rise in places such as Europe.
Also, a layperson should never be performing their own rites. Only the designated clergy should do that.
This isn’t something we should ever be concerned about beyond everyday practical things that every person ought to be doing anyway: receiving the sacraments, abandonment, prayer, thanksgiving, penance, and spiritual communion with Mary, our Angel, saints, etc.