the most usual way is when you have participated in some conferences in the past (most of them come as series every 2-3 years). That means that your email address is included in participants' lists which are then used by upcoming conference committees to attract delegates.
Another way is if you are a member of big societies related to your research fields of interest. Then you get almost daily announcements about seminars, conferences etc.
Yet another way is through university group email announcements. Several university professors and senior researchers are receiving such announcements and calls for papers (according to the very first way I mentioned) and they usually forward these emails to colleagues, phd students and other researchers.
Finally, you can always manually browse the internet and target specific webpages of institutes and societies related to your research fields (as mentioned in the second way).
I hope I answered your question. If you need to ask further I am at your disposal.