You’ve got to look after the future of your club. Your Reserve team is always available for viewing and for interaction from the tabs menu and 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 them 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 the Reserves.
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.