How much did Europeans know about the black death?

Actually, quite a lot. When the plague first arrived in Italy, various communities of physicians collaborated to try and work out what the disease was and how it affected the body. This involved the authorisation of autopsies by the papacy, and collation of findings in a way that medieval Europe hadn't done before. Similar efforts were underway in the kingdom of France under the king's supervision, but he did not authorise autopsies and worked more from conventional wisdom than in-the-field efforts. To give them their due credit, the French were trying to work out what to do before it got there, which somewhat limited the information they had available.

The findings of the Italian autopsies survive through a letter sent from the papacy to Bruges, warning them of what was coming:

It is said that the disease takes three forms. In the first people suffer an infection of the lungs, which leads to breathing difficulties. Whoever has this corruption or contamination to any extent cannot escape, but will die within two days. Anatomical examinations, in which many corpses were opened, were carried out in many Italian cities, and also, on the pope’s orders, in Avignon, to discover the origins of this disease, and it was found that all those who died suddenly had infected lungs, and had been coughing up blood. And this form is the most dangerous of all these terrible things, which is to say that it is the most contagious, for when one infected person dies everyone who saw him during his illness, visited him, had any dealings with him, or carried him to burial, immediately follows him, without any remedy.

There is another form of the disease which exists alongside the first one, in which boils erupt suddenly in the armpits, and men are killed by these without delay. And there is also a third form, which again co-exists with the other two, but takes its own course, and in this people of both sexes are attacked in the groin and killed suddenly. Because of the growing strength of this disease it has come to pass that, for fear of infection, no doctor will visit the sick (not if he were to be given everything the sick man owns), nor will the father visit the son... unless, that is, they wished to die suddenly along with them, or to follow them at once. And thus an uncountable number of people died without any mark of affection, piety or charity – who, if they had refused to visit the sick themselves, might perhaps have escaped.

So they correctly worked out it could cause both buboes (which we know today to be an infection of the lymphatic system) and a respiratory infection. They realised the lung infection was ludicrously deadly and infectious, something we in the modern day didn't actually realise until the 20th century, and even then it wasn't commonly accepted in the west until 2014 when it broke out in Madagascar! It has a 99% mortality rate and kills in under 48 hours, so yeah, pretty deadly. They were slightly wrong about the bubonic form of the disease, as this does not 'kill without delay', it usually takes about a week.

They also knew it was a contagious disease, and although they lacked the tech to know about bacteria, they did realise that it spread through mutual contact with items and by sharing a space with an infected person. Bruges and its surrounding towns took the contagion aspect of the disease on board, and managed to slightly reduce the scale of death by severely limiting public gatherings and closing the gates to new visitors. Of course, they had no knowledge of how to treat the plague, so all they could do was slow its spread.

Did they know it was happening in other parts of the world?

Yes, and they realised that pretty quickly. The plague first arrived on ships travelling from Crimea to Italy, so they knew immediately that it must be affecting the people in the southern Steppe. A contemporary, writing in the early 1350s, noted that:

In 1346, in the countries of the East, countless numbers of Tartars and Saracens were struck down by a mysterious illness which brought sudden death. Within these countries broad regions, far-spreading provinces, magnificent kingdoms, cities, towns and settlements, ground down by illness and devoured by dreadful death, were soon stripped of their inhabitants.

Did they know once you got bubos it meant certain % of death?

As the papal autopsies discovered, the buboes actually weren't the major threat. It was the lung infection. This is true even today - the bubonic infection, which is contracted from flea bites, is slow enough that antibiotics do a very good job of treating it, but the lung infection needs antibiotic treatment as soon as symptoms show or they will be dead within a day or two. They believed the Black Death meant certain doom, but that wasn't really the case. The lung infection meant death, but the bubonic form itself only killed about 50% of the people who got it. We know from tax records that in a dense population with both forms spreading, about 70% of the city will die. That's... a lot, but it's also not certain death. Contemporaries did realise that if you just avoided the sick and shut yourself in, then the odds of survival were actually pretty good, but people still had to leave to get food, go to church etc. so there was a limit to how successful self-isolation attempts were.