I cannot speak for all JV teams, But I can say in the NCAC that most teams have them. There are no rosters for JV teams. In all the cases I know oif they practice and lift with the varsity. Most players do not know they are going to play a JV game until the week of the game. The head varsity coach coaches just like he would for varsity. He may give his younger assistants opportunities to be base, or bench coaches during these games.
Recently OWU played Dension in a DH. I know of 3 Varsity contributors (not necessarily starters) played. Pitcher were week day starters. The JV will play 2 or three games over spring break. After they get back from Florida there will probably be a couple of games during the season.
As far as progressing from JV to varsity. The majority of my sons team that played in JV games, eventually made it to varsity. If you were not playing on Varsity or JV, the chances of you ever playing varsity were slim to none. Now playing on JV was not a requirement to play Varsity. Many players at my sons school never played a game of JV, but most of those started Varsity as Freshmen.
When my son went through the process, none of the coaches ever mentioned JV teams and we never asked.