The purpose of this study is to evaluate safety and efficacy of risankizumab in participants with ulcerative colitis (UC) in participants who responded to induction treatment with risankizumab in a prior AbbVie study of risankizumab in UC. This study consists of three sub-studies and a Continuous Treatment Extension (CTE): Substudy 1 is a 52-week, randomized, double-blind, placebo-controlled maintenance study; Substudy 2 is 52-week, randomized, exploratory maintenance study; and Substudy 3 is an open-label long-term extension study for participants who completed Substudy 1 or 2, or participants who responded to induction treatment in Study M16-067 with no final endoscopy due to the Covid-19 pandemic or due to the geopolitical conflict in Ukraine and surrounding impacted regions. The CTE is an open-label extension for Substudy 3 completers to ensure continuous treatment with risankizumab until such time that risankizumab becomes commercially available and/or the subject can access treatment locally.