Group B strep is a normal part of our skin flora, about 1/3-1/4 would test positive for it at any time. It only causes an issue in adults when it makes its way into your bladder and can cause a UTI. You'll get antibiotics for GBS in your urine, but not on a swab as it is normal to find it in the vagina. It can, however, cause some babies to become very unwell - working in our own neonatal unit, I have seen babies who have GBS infections that need a lot of help. If you test positive at any time in pregnancy, you'll be offered antibiotics through a drip in your labour to protect baby. If you have had GBS with a previous pregnancy, you may be offered the choice - this is different in some different areas. Your baby will also receive some monitoring after birth - normally that's 12-24 hours of observations every 2 hours (heart rate, temperature, breathing rate). There are some things that make your baby more 'at risk' such as having your waters gone for over 24 hours, being very small, having a premature baby or you being unwell in labour (having a temperature yourself). If baby has 2 'risk factors' (GBS is one, the things I've mentioned and others are the second), or there are any concerns for your baby after they are born, your baby's doctors may want to start antibiotics for baby through a drip in their hand (although this might be different at your own hospital). Usually this is for between 2-5 days, but it isn't just for suspected GBS infections that babies receive this.
Some people have babies who are unwell with GBS without it ever being picked up on their swabs/urines, despite having them. Most people who have GBS+ve results and receive antibiotics in labour will have no issues. I had antibiotics for it with both of my vaginal births (both premature) and no issues.