there are at least 3 places to mention your undergrad research experience:
1. your personal statement
2. your scholarly activities (there is a section on ERAS for research)
3. under the "Publications" section
if you're interested in academic medicine, consider mentioning the research in your personal statement as demonstrative of an early commitment to scholarly activity; if you're not interested in academic medicine, use the limited space in your PS to talk about other things
i would definitely mention it in the extracurricular/research section of the application
you have to decide whether to put it in the publications section. a rough guide: if you can find it on pubmed, list it as a publication. otherwise, you can make a separate entry in the research section for any presentations/posters you did related to that project
there's no right answer to this question. the only thing at stake is annoying residency committees by putting in extraneous information. if the research was important to you and you can talk about it intelligently at an interview, by all means put it in your application. undergrad experiences should not be left off the application just because they happened before med school.