How do I smooth concrete walls?

Where do you live?

Go to your local home improvement store, their associates are knowledgeable in those things and they should be able to help you with what you need to smooth out your walls.

https://www.hunker.com/13402579/how-to-plaster-a-concrete-wall

https://www.reddit.com/r/DIY/comments/3dls98/how_best_to_make_an_interior_concrete_wall_smooth/

There is a masonry stucco made by most paint manufacturers that would work for you. They're sold at Hardware Stores.

Joint compound; just "smear" it on in a pattern that appeals to you, using a trowel of some sort. You can get fast drying which is lighter weight, but the regular would likely be more durable. Probably a good idea to seal the concrete first to block any damp.

You can re-coat with a concrete product which will be available at your local hardware store. They may also have some other ideas on how you can accomplish your goal.

If they are interenal walls plaster of paris is a good option. You can read more about plaster of paris - https://happho.com/gypsum-plaster-advantages-disadvantage/. But incase you are either not interested in Plaster of paris or you want to plaster in wet areas/exteriors, then you may use cement sand plaster. This is just a mixture of cement and sand in 1:4 proportion.