Short answers, pulled from the story.
Japanese armour first appeared in the 4th century, as evidenced by the discovery of cuirasses and basic helmets in graves. These early pieces evolved from ancient Chinese designs before developing distinct local characteristics.
High-ranking mounted samurai wore heavily built o-yoroi suits requiring up to thousands of kumihimo braided silk cords in various colours while lower ranking foot soldiers typically used lighter d-maru versions instead of the elaborate mounted warrior gear. The artistic decoration reached its peak around the Genpei War between 1180 and 1185 when a single suit might require extensive weaving work lasting many months.
Japan began trading with European powers principally the Portuguese Empire known as Nanban trade after matchlock guns arrived on Tanegashima island in 1543 which led to creation of tosei-gusoku modern armours designed from earlier d-maru templates while incorporating bullet-resistant tameshi gusoku testing methods. Samurai needed much lighter armour because mass-produced tanegashima firearms changed battlefield dynamics completely.
The last actual use occurred in 1877 during the Satsuma Rebellion when samurai fought against government forces. When unified Japan entered the peaceful Edo period spanning 1603 to 1868 traditional battle gear transformed into ceremonial status symbols rather than functional equipment.
Oyamazumi Shrine houses forty percent of all Japanese national treasures designated as Important Cultural Properties according to government records. Kasuga Grand Shrine maintains another large collection of valuable historical armours including Kamakura period o-yoroi pieces dating between thirteenth and fourteenth centuries.