Dolmens are all over the place in western Europe, particularly in Ireland and England. You can see how they’re designed to look like a big table or a sheltered doorway or portal. Neolithic dolmens were usually (they think) tombs, with the ashes of people buried inside. Sometimes they were then covered over by earth and made into a mound. It’s tempting to climb around on them and inside them, and picture how they looked before they collapsed or were damaged.
The best guess is that they piled up dirt and logs and rolled the giant stones up into position, then removed the props to allow them to settle onto each other. Some of the stones are incredibly heavy. Sometimes these monuments are at the top of a big hill. It’s mind-boggling. Why? How? This was all long before there was written history, so we are left to our own imagination.