You’ve got to look after the future of your club. Your Reserve (or junior age equivalent, e.g. U21) and Youth teams are always available for viewing and for interaction from the tabs menu. Your Youth Team will typically contain teenage players on youth contracts training and trying to earn a professional deal. The Reserve Team typically may contain a mixture of young professionals and older, ‘washed-up’ players who no longer cut it at the senior level, depending on the country you’re managing in. How you manage these two teams is completely up to you.
At the start of each season you will be asked by the governing football association if you wish to continue entering/enter the competitions for these teams.
B Teams, however, often exist as almost separate entities and are teams competing in professional competition in their own right. As first-team manager, you of course reserve the right to be able to move players about as you see fit but they will often have their own budgets, their own transfer policy, and the rules governing when and how you can move players between squads may be different from league to league. Check the League Rules screens for specific information.
You can designate a member of your backroom staff to control these teams in your Staff Responsibilities.