iPhone SE 3 not syncing with iTunes

Iphone se3 running IOS 18.2.1 is visible on my iMac running High Sierra but will not synch. In the devices pull down on iTunes, synch appears as an option but is not live. iTunes has downloaded purchased albums but I have multiple albums I have loaded into iTunes on the iMac that I need to transfer onto Music in the iPhone, Please advise.

[Re-Titled by Moderator]