How is CRM customization done?
Generally speaking, CRM customization usually falls into three broad categories. First, custom fields are added to CRM entities, like leads, contacts or opportunities in order to collect additional information that default forms don't allow.
The second type of customization involves using existing sales and marketing automation tools in order to create specific scenarios that are relevant to the client's business.
The third type of customization involves creating custom integration with non-CRM tools and services. For example, integrating CRM with accounting software.
If you are looking for a CRM system that provides you with a wide range of customization options, consider Bitrix24 and its on-premise edition that comes with open-source code.