Studying family history from online sources.
I got interested in genealogy in 1993 when I received copy of research made by my mother's brother. He did research both my father's and mother's family.
When I started my own research, I noticed some errors in the original research. Some dates were obviously invalid and there was some generations missing. I did detach some persons from their parents so I could look up various databases for correct parents.
As I was missing birth dates and names for the parents, I made rough guesses that parents were 25 years older than their first child. Some entries for the children already contained patronymic which is basically genitive form of father's name. Very few names of children contained matronymic which is on the other hand genitive form of mother's name. Using that information I could look up enough data from online sources so I could again search for the incorrect parents within direct ancestors and close family members. After a while, I could fill in the correct parents and eventually found how they were related to the incorrect ones.
More I guessed missing details, more I could compare with other sources to fill in missing information. Sometimes I had to adjust birth dates multiple times before online search engines could find possible matches. As some of the profiles were private, I had to use tricks to bypass the checks to get the information...
Using MyHeritage Family Tree Builder, it is possible to see private information from some profiles after user syncs with the online version. When the profile is open in website, it will show person's name, birth date and death date. That is usually enough to get more information revealed after resyncing again.
Further back in time, there is a lot of lineages where both father and son had same names, so sometimes a grandfather was linked as person's father instead. There was also people who had children before they turned 18 years old, but that was expected as during those years, people did die a at younger age, a lot of them didn't live older than 40 years.
When I started online research, I didn't use any special software, just free versions of genealogy websites as they allow limited access to research made by other users, even from different websites. When using research of other people as background information, it is essential to remember that any research can contain errors and must be verified from official sources if available. Wikipedia and personal websites of descendants are not considered official sources.
There is some published books about noble and royal families but some of them are written in other language than English. When I started researching, I had to read English, Swedish, German, Polish, Latvian and Russian to find enough information about my own family.