A frequently asked question is how to find the target universities. This issue has to do with many factors. We are now talking about the United States, and you can generalize about the rest of the world and use the same method.
Note: The number of five universities to apply for is just one example, and you can apply for more or less. There have been students who have applied for two universities. Some have applied for 20 universities.
If you are applying, this article is about how to find the target universities and the experiences will be useful for you.

first step
Extract the list of all universities in your field from reputable ranking sites such as US News. Just go to the link below and look for your favorite field:
https://www.usnews.com/best-graduate-schools
If for some reason you did not find the field you are looking for directly, another way is to use Google and search for the following phrase:
List of graduate programs + field name + country name
Example: Graduate programs in biotechnology in the U.S
The following link is one of the good lists for senior:
https://www.stoodnt.com/blog/best-universities-for-ms-biotechnology-in-usa/
And the following link for PhD:
https://www.phdportal.com/disciplines/118/biotechnology.html
Step two
Prioritization of universities based on ranking: Suppose you only want to apply to 5 universities. The recommendation is if:
1- Your resume is very excellent (GPA 18 and above, TOEFL excellent, strong research and good recam) Choose two universities below the rank of twenty, two universities ranked from twenty to 50 and one university around the rank of one hundred. The fifth university should be almost certain to admit you as a precaution.
2- If your GPA is around 15-16 to 18, your language is between 90 to 100 TOEFL and average research or you do not have a university under twenty, one between twenty fifty, two to fifty to one hundred, and finally a university around 150 rank for caution Select.
3- If your GPA is around 13-15, there are three universities between 20 and 100, one university around 150, and one university around 200, or a university that is very small and has a high chance.
Note that these combinations are relative, and you can make more or less choices based on risk, interest, and cost. There were people who with a grade point average of 16 and TOEFL 90, only two universities applied around the rank of 110 and were accepted from both. So it is not a general rule. The goal is to get an idea of how to choose.
How to find target universities
Third step
Detailed search of the faculty site (ies) related to your field in the selected universities. The best way to do this is to do a Google search to find the admission requirements of the relevant university:
Graduate admission requirements + University name + Faculty name
If you have the conditions for university admission, go to the fourth step, otherwise the second step and find an alternative with a similar rank for this university.

Step four
You have now prepared a list of various universities whose admission requirements apply. It is the turn of finding related professors. Do a Google search again:
Faculty + name of university + name of faculty
With a little searching, you will find a list of all the faculty members. Some colleges list professors based on research topic. However, save the names and emails of professors associated with their personal page or lab link in a Word or Excel file. If no professor at your university is interested in you, take the second step to find an alternative.
Fifth step
Use instructions 1 and 2 to prepare resumes and text emails to professors and start emailing. Record each professor you email to the Excel spreadsheet. Preferably if you are emailing more than one professor from the same university, it should be two different days so that your email is not recognized as spam.
After ten days to two weeks of each email, remind the email again with the subject: Follow up Ph.D application.