Many people write fanfiction. Many people also read fanfiction. Generally, such people are fans of the original work.
Generally, yes, but not always.
Fanwork-Only Fans are fans who produce and/or engage with fan-made media without being overly familiar with the source material, if they even care much for that original work to begin with. There can be any number of reasons for this. For instance, someone could be introduced to a fandom at a time when the series in question is not readily available in their language, so engaging in fan material became the best way to partake with the work before becoming the only way they partook. Alternatively, someone might have been attracted to the fanwork scene due to it highlighting some aspects of the original work said person enjoys the most, such as the setting or a certain character, thus leading to said works overshadowing the original for them. Especially if that fan feels that the original has undergone Seasonal Rot. And in some cases, someone might have actually been introduced to the franchise through its fanwork first and never take much interest in the franchise beyond that.
For some reason, this tends to show up more often in works that have lots of male characters, though works with a similar setup of many-to-improbable levels of female characters attract just as much attention. Needless to say, this can easily result in copious amounts of Fanon, as well as Canon Defilement in worse case scenarios. Naturally, expect to see this type of fan declare many fanworks to be Better Than Canon.