Round brackets in a pattern create a group, capturing that part of the match. A match object gives you the groups:

import re

m = re.search(r"(\d{4})-(\d{2})-(\d{2})", "Due on 2026-10-07, thanks")
print(m.group(0))
print(m.group(1), m.group(2), m.group(3))
print(m.groups())

print(re.findall(r"(\w+)@(\w+\.\w+)", "ada@example.com, alan@test.org"))

group(0) is the whole match; group(1) onwards are the groups. When a pattern has groups, findall returns the groups (as tuples if there's more than one) instead of the whole matches.