Extracurriculars for college apps

On college applications, how do you indicate extracurriculars? Is it just a listing of things you’ve done (school clubs, internships, community service, etc)? And do I have to provide record/proof of each extracurricular? For example, some community service that I have done is signed off on a sheet that my school has record of (for an award at graduation), but a lot of volunteer work I’ve done I unfortunately don’t have written proof of. Would I still be able to list these things on my college applications?
Also, what exactly would versus wouldn’t count as ‘extracurricular’ because I know some things that certainly do but I feel I’m missing some things that would count.
Lastly, how big of a factor are extracurriculars for college admissions? I know it differs for every school, but on average?

No

Yes

If it’s an activity outside of the school day, it’s an EC.

As you said, depends upon the school. Generally more important for private unis than public, and the more selective the university, the more important the EC’s are. Is there a magic percentage number? No.

CSUs don’t care in the least, while a private, religious school may give them substantial weight (usually as discussed in your personal statement, not just by listing them). UCs are in between but, really strong ECs won’t make up for a big GPA/SAY deficit.

The best way to keep track of your volunteer activity is by maintain a list - noting the name of the organization, the date and who was in charge of the activity. If you are asked to validate your claims, you have plenty to go back to the people you helped