Short answers, pulled from the story.
A stupa is a domed hemispherical structure used in Buddhism to enshrine sacred relics, including the physical remains of Buddhist monks or nuns, objects belonging to the Buddha, and sacred images. It serves as a place of pilgrimage and meditation. The word first appears in writing on Ashoka's Nigali Sagar pillar, inscribed in Pali in the Brahmi script as thube.
Buddhist stupas are classified into five types based on form and function: relic stupas, which enshrine the remains of the Buddha or saints; object stupas, holding items that belonged to the Buddha; commemorative stupas, marking events in the Buddha's life; symbolic stupas, representing aspects of Buddhist theology; and votive stupas, built to commemorate visits or gain spiritual merit.
Borobudur in Java, Indonesia, is acknowledged by UNESCO as the largest Buddhist monument in the world. Built in the 8th century, it features rows of bell-shaped stupas on its upper terraces and a central main stupa left deliberately empty to symbolize complete enlightenment.
The stupa evolved into the pagoda through Gandhara, where the form gradually grew taller over several centuries, culminating in the towering second Kanishka Stupa of the 4th-5th century CE. That tower form is considered the direct precursor of the Chinese pagoda. The Asian words for pagoda, including ta in Chinese and to in Japanese, all derive from thupa, the Pali word for stupa.
Pradakhshina is the ritual of walking clockwise around a stupa. It has been a devotional practice in Buddhism since the earliest times, and every stupa is built with a pradakhshina path. The Buddha himself left instructions for paying homage at stupas, which include laying wreaths and offering sweet perfumes and colours with a devout heart.
The Benalmadena Stupa in southern Spain is the tallest stupa in Europe at 33 metres high. It was inaugurated on the 5th of October 2003 and was the final project of Buddhist master Lopon Tsechu Rinpoche, who built 16 stupas across Europe before his death in 2003.