My baby likes to have her arms out so we swaddle below the arms rather than tuck them in. She sometimes gets too cold though as she inadvertently pushes the blanket down so her chest is out of the blanket. How do we keep her warm enough?

Dress baby and use layers as you would dress or use layers yourself: to be comfortable, neither too hot nor too cold.
Consider using a safe baby sleeping bag (one with fitted neck, armholes or sleeves and no hood).
Dress baby for sleep and add/remove lightweight blankets to ensure baby’s back or tummy feels comfortably warm to the touch.
https://rednose.org.au/article/bedding-amount-recommended-for-safe-sleep

There is also no evidence to support maintaining a specific room temperature or any specific bedding configurations (number of blankets required) as this depends on a number of factors such as what the baby is wearing, whether it is summer or winter and whether there is heating or cooling within the room where the baby is sleeping.
https://rednose.org.au/article/room-temperature