I've only played with Westin, DeeZee and Manik. They have what looks like brass and aluminum inserts or just have threaded bar inside the end of the tube.. Usually the brackets are fitted to select vehicles, so no adapters are needed. Just insert some speed nuts, hold the bracket up and run a bolt in. Then bolt the bars to the bracket.

Now another thing to look at is polished Stainless or chrome plated. Polished will most likely be bare and will need to be regularly waxed to keep them shiny.