Dear Irfan Madani,
Good day!! Thanks for positing your concern in this Microsoft Community.
Per the description shared, I would like to summarize your concern i.e., you are finding for the Form Settings in the SharePoint Online list so that you can change the default form in the list.
If my understanding is correct, I would like to convey that Form settings is not available in the Advanced Settings of the List Settings, where it's visible in the List Settings as the main setting:
If you are not able to see the Form Settings under the General Settings, can you please check whether the Form settings is available for different list in the same SharePoint Online site? If yes and the problem is only with specific list, can you please provide the name of the list template you have chosen for creating the list?
Thanks for your patience and understanding.
Sincerely,
Mia | Microsoft Moderator