ASP.net Core Identity Sign in as another user from Administrator

You can use the .SignInAsync() method from SignInManager to log in as a specific user without knowing their passwords:

public class ImpersonateUserHandler : IRequestHandler<ImpersonateUser, CommandResult>
{
    private readonly SignInManager<AppUser> _signInManager;

    public ImpersonateUserHandler(SignInManager<AppUser> signInManager)
    {
        _signInManager = signInManager;
    }

    public async Task<CommandResult> Handle(ImpersonateUser command, 
        CancellationToken cancellationToken)
    {
        try
        {
            cancellationToken.ThrowIfCancellationRequested();

            // Look up the user you're going to impersonate
            var user = await _signInManager.UserManager.FindByIdAsync(command.UserId);

            ...

            // Login
            var signInResult = await _signInManager.SignInAsync(user, false);

            ...
        }
        catch(...)
        {
            ...
        }

        ...
    }
}