If you want bougainvillea to keep blooming nonstop, you have to prune it hard and promptly after the flowers fade.
On my home balcony I have a bougainvillea 'Zhonghong' (Bougainvillea spectabilis 'Zhonghong'). It started blooming this summer and has not stopped since. The pot is neither too big nor too small, and I did not withhold water. The main methods are: fertilizing (a compound fertilizer bought at the flower market) and pruning. Once the flowers start to look a bit spent, prune immediately. Do not feel sorry for it; cut a bit more, then fertilize once a week with a weak solution, and water generously every morning and evening. Because there are relatively many branches and leaves, they do not all bloom at the same time: some flowers are blooming vigorously, some are starting to fade, and some are just budding. So after pruning, new flower buds sprout right away, and that is when you get more blooms.
One thing to note here is that small plants are not suitable. In summer and autumn, large plants are absolutely fine, but during the low-temperature periods of winter and spring they need dry management. In fact, the so-called large plant is just a 'Zhonghong' on the balcony with relatively many branches that has grown for more than 2 years. The key is to cultivate more branches and let them climb along the balcony's security netting. With many branches, you can prune frequently, and the flowers will bloom in staggered waves, which can achieve the effect of nonstop blooming.
Most bougainvilleas (on the same plant) bloom at the same time, fade at the same time, and are pruned at the same time, so before they bloom again there is an interval with no flowers. If you follow the method above, as long as it has bloomed once, it will keep blooming nonstop. Of course, this is more suitable for bougainvilleas with lush branches and leaves. For small ones, use pruning sparingly, otherwise it will become a "one-cut plum" (a plant that is cut back and never blooms). This method is only suitable for the southern Guangdong and Guangxi regions and the Hainan region, and the bougainvillea must also be one with relatively many branches.