> When no one owns copyrights - everyone does. Both you and you custom have full rights to copy and distribute those parts of software as do everyone else, you just don't own exclusive rights (copyrights) to control whether and how anyone else can also copy those parts of software. Do you own copyright for number "10", does it mean you can't use it in your software.

Yes. It can be an issue depending of the wording of your agreement with the customer. For example, if 'you' agreed to develop a piece of software 'exclusively' for the customer, and then use AI to create substantial parts of the software, then neither it was 'you' who developed that, nor was it 'exclusively' for the customer as you can't grant exclusivity.