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You can scrub the pots still using diluted bleach water and remove any old soil and decay.
Allow the pots to air dry after cleaning.

https://www.gardeningknowhow.com/special/containers/how-to-clean-containers.htm

Depending on the variety of Hydrangea, they can become quite large.

These article will help you with your decision and care.

https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/hydrangea-container-care.htm

This could all come down to the exact climate zone you are in. If you are ever in doubt, then it is better to bring the plants in for the winter.

You have several different options here, but the good news is that it is fairly easy to give these winter protection. This article will give you the information you need: https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/hydrangea-winter-care-how-to-protect-hydrangeas-from-winter-cold-and-wind.htm

You may want to feed your plant with a phosphorus fertilizer such as bone meal. It may also be that it was pruned incorrectly. This article explains the problems: https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/hydrangea-not-blooming.htm

You will, definitely, need to change out the soil. It is likely too rich and disease ridden from the soak, now.

I would opt for a mix of potting soil and compost. Make sure to follow all instruction, carefully, to avoid overfeeding, and only water when the top 3 inches of soil is completely dry. This will help to prevent overwatering. In fact, after watering, take the inside container out and dump off any extra standing water in the second container.

Here is an article that will help you to care for the shrub in container:

https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea/hydrangea-container-care.htm