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Those are most likely Yuccas but they are not the same as your houseplant yuccas. They can take much more cold and they also bloom. You should be able to find them at your hardware store in the outdoor plant section. Their common name is Adam's Needles and they are rather popular because they need little care or water. They are pretty much care free once you plant them.

Make sure your plants have adequate drainage. Typically, fertilizing is not necessary for these plants. Also, if they have gotten too large, they may require pruning. The following articles should be of some help to you:
https://www.gardeningknowhow.com/ornamental/foliage/yucca/caring-for-and-landscaping-with-yuccas-outdoors.htm
https://www.gardeningknowhow.com/ornamental/foliage/yucca/yucca-plants-care-and-pruning-tips-for-pruning-a-yucca.htm

If the stems are still alive, they will regrow their leaves and there is also a chance the roots are alive and they will regrow from the roots. I would give them a month, and if you do not see growth by then, they are most likely dead. These articles will helpyou as well:
https://www.gardeningknowhow.com/plant-problems/environmental/how-to-tell-if-a-plant-is-dead-and-how-to-recover-an-almost-dead-plant.htm
https://www.gardeningknowhow.com/plant-problems/environmental/tips-for-saving-cold-damaged-plants.htm
https://www.gardeningknowhow.com/ornamental/foliage/yucca/yucca-cold-weather.htm

The following articles should be of some help to you:
https://www.gardeningknowhow.com/ornamental/foliage/yucca/separating-and-repotting-yucca-offshoot-pups.htm
https://www.gardeningknowhow.com/ornamental/foliage/yucca/propagation-of-yucca-plant.htm

This article should help with that: https://www.gardeningknowhow.com/ornamental/foliage/yucca/yucca-cold-weather.htm

Brown tips can happen if a plant is over or under watered. Likely too much water with the high humidity in greenhouses. Now that it is outside, it should be fine. Water it when the top one inch of the soil is dry.