Demons regularly attack everyone through temptations to sin. That is their "ordinary activity", if I may use the terminology of the RC Fr Gabriele Amorth (an exorcist for the Vatican). If you've ever committed a sin in response to a temptation, a demon fought you and momentarily won. A succession of such victories may weigh us down enough that they gain a foothold in our lives through our lack of struggle against them and our hardening in sin. That in itself may be all there is to it, but it is also possible for what we commonly regard as "demonic activity" to manifest itself in such circumstances. This can happen "just like that", but more commonly there might be some involvement with the occult or some other "escalation" that brings it on. This manifestation of demonic activity Amorth calls the demons' "extraordinary activity".
But when a person turns away from sin and turns to the Lord decisively, whether or not that involves embracing monasticism, demons will fight harder to win him back because the person himself has begun to fight against them with the power of God. In the majority of such cases, it's probably just an escalation of temptation, but in particular cases there may be some "manifestation". The goal is the same: to entice, to frighten, to ensnare, to capture.
It's important that we know a little about these things (most likely, what is in Scripture is enough), but also important that we not "glorify" it too much. Christ has conquered the power of Satan. If Satan acts, he nevertheless is always subject to the power of God. He acts only within the limits God allows, for the time that God allows, etc. When it's time to go, he goes. And we have weapons: prayer, fasting, the sacraments, almsgiving, ascetic practices, and so on. If we live a faithful life within our abilities, not getting too lazy or strict beyond what we can handle, we'll be fine. We're in good hands.