To board a plane with a baby you can carry the baby through security, in a carrier or in your arms.
Most airlines will allow families with young children and babies to board the plane early to get settled in.
If you're boarding the plane with a baby and have a car seat or a stroller you can also check the car seat or stroller at the gate for free or even at the check in counter.
If your baby is under 2 years old they can often travel with you on the plane as lap infants without needing a separate seat.
Although you have to confirm this with the specific airline and may need to pay a small fee as it can vary depending on the specific airline.
When by babies were under I simply had them with me as lap infants without a separate seat.
2 years and above will need a separate seat and likely a car seat or child restraint as well and you will have to pay a separate fee.
When bringing your baby through airport security you will have to remove the baby from the stroller or car seat and carry the baby through the metal detector.
TSA also allows babies to be carried through the metal detector and they may also be subject to additional screening.
A sling or carrier can also be used to hold your baby through security.
During the flight if your baby is boarding on the plane with you as a lap infant the baby will sit with you on your lap during the flight.
You may also be given a special lap belt which is to attach to your seatbelt and if you purchase a seat for your baby you can also use an FAA approved car seat on the plane.