I visited secure units as part of a previous job. It is very much like a prison for children. Rooms are locked up at night and those rooms that very much resemble a cell with the hatch on the door, bars/grates on the windows.
Education happened in the unit usually in a separate wing or part of the building with classrooms. Outdoor time was very strictly monitored in an enclosed yard. They have social/recreational spaces but again heavily supervised.
Young people arrived by van straight from court and into the unit via a garage, fully searched on arrival and placed in a holding-type empty cell.
Visiting was very similar to prison visiting arrangements, depending on the level of security for that young person i.e. those who were transitioning to open units may get to go 'out' for a couple of hours, others can't leave the visiting room and were fully supervised.
Some young people were there on mental health grounds if they are a danger to themselves and can't be accommodated in hospital. One girl I worked with had a severe eating disorder and was on suicide watch.
Generally they are not happy places. The staff were very dedicated, kind and cared a lot about helping the young people but it was really oppressive and felt like such sad places, filled with trauma. There is no escaping the locks on the doors and lack of knives at meal times, no matter how many nice posters you display or craft workshops they put on.
Google image search 'secure unit' for an idea of what they look like inside.