You can acquire new players for your regular squad by recruiting youth players. This can be done in two different ways:
Recruiting a player from the outside directly into your senior squad by using your scouting network.
Starting a youth academy, letting you recruit talents and develop them. You can play matches against other youth teams in private or public leagues, and eventually promote your talents to the regular team.
Promoting a random player from the outside to your senior squad is a simple and effective way to acquire new players. Running a youth academy is more challenging, but may also be a lot more fun.
You can only use one system at a time. If you are running a Youth Academy, you cannot promote a youth player from the scouting network. However, your academy costs also counts as scouting network investments, making it swifter for you if you want to close your academy and use the scouting network instead.