Disclaimer: I am a PhD student with some outside knowledge (as an applicant) and some inside knowledge (admissions committee student member) on an average doctoral admissions process.
There are a number of factors which affect a candidates acceptance (or conversely, rejection) into a PhD program. They are standardized test scores, GPA, research experience, publications, letters of recommendation and the personal statement.
In my opinion, from what I have seen your numbers (GRE and GPA) don't do a good job of getting you in but can keep you out. What this means is that you have to clear whatever internal cut-offs there are. This can be subjective or objective depending upon the department, the field and the university. For instance, this could be 3.5 in one department in one field or could be 3.8 in another department in another field in another university. Standards can also be relaxed for candidates with exceptional research records and letters of recommendation written by trusted researchers.
The most important factors which matter are previous, relevant research experience, letters of recommendation, personal statement and publications. Some optimum combination of this must convince the admissions committee that you are worth admitting.
In fact, I would venture to say that there isn't much of an impact in graduating early or transferring to and from different programs. Focus on selecting whether a research career is right for you and what field you actually want to get into for graduate school.
The second part of your question is a little broad as I have noted in a comment. If you search this platform, you will find numerous relevant advice from folks much more knowledgeable than me. There are different approaches here. Some people just know what field they want to get into from the very outset. Others (like me) bounce around in different fields until one feels very right to them. Yet others get into a masters degree program to try out a discipline in terms of exploring research before committing to a full time PhD program.