Are these actually Russian?

No. Not a single title in the archive says Russian, and no Russian-marked example turns up in the evidence. The heading covers the English tea or hot-water urn, which did the same job by a different method: a hot iron billet or a spirit lamp instead of a charcoal chimney. The two names are traded as one.

What are most of them made of?

Copper. It appears in the majority of titles, usually as a plain body with brass mounts, tap and handles. Beyond that there are four silver-plated urns, two japanned Pontypool ware urns, and a handful of Dutch pewter and toleware coffee urns.

The dated field runs from 1800 to 1920, with 24 of 58 listings carrying a clean year. Much of the dating is broad rather than precise. "19th Century" is the largest bucket and "1800s" the next, while Regency, Georgian and Victorian are reign labels rather than dates. Treat any single year as the dealer's estimate unless they say otherwise.

How many come up for sale?

Around seven a year on Sellingantiques, spread across 21 different dealers. That means most dealers who handle one handle only one, so the category is thinly supplied rather than steadily stocked.

Does anything unusual turn up under this heading?

Occasionally. A Japanese cloisonné enamel temple water dispenser with a dragon, 60cm tall and dated circa 1880, is the largest and highest-priced object in the archive, and it sits alone at the top. At the other end, a Kashmir copper-and-bronze snake samovar is the only piece from the Indian subcontinent, and it sits at the very bottom of the range.

Why do listings say samovar and tea urn together?

Because dealers use both words for the same vessel, and buyers search on both. There is a run of near-identical listings titled "Antique English Georgian/Victorian Copper Samovar", and "tea urn" recurs across the category, frequently in the same title as "samovar".