What they want and need is a land link between Russia and the Crimean Peninsula-—A rail line from Moscow to the Black Sea. The shipyards in southern Ukraine would be handy too. They do not need all of Ukraine, just a bit of it. And they will get it—how? Hard to say. But they will not just roll the T-90s in. They will do it when the USA is distracted—by political instability or Civil War—or a war with China. But, Russia will need to make a deal with Germany and Poland first.
Putin will not be satisfied with just a land link. He wants a subservient Ukraine controlled by Russia.
That’s dated take. There is a land link with Crimea in form of the Kerch bridge. All the shipyards in Ukraine went bankrupt since 2014 and cut for scrap. Russia over that period developed its own shipyards on Batlic and in the Far East, capable to build anything. FYI in Q3 2020 Russia surpassed China as a second biggest shipbuilding nation per GRT in a moment.