With Magento announcing Magento 2 way back in 2011 the move from Magento 1 is someting that has been a dicsussion for a while. The first official release of Magento 2 turned out to be a backwars incompatibile release with the Magento 1 version, as the framework went through a major rewrite, leaving the merchents with Magento 1 stores with a bumpy upgrade process.
Magento has informed that Magento 1 security support will be discontinued in June 2020 https://magento.com/blog/magento-news/supporting-magento-1-through-june-2020. So you decided that you want to move from Magento 1 to Magento 2 and need your store migrated to the latest Magento 2 version.
Migration from Magento 1 to Magento 2 can involve and combine different parts of your current store:
Migration of a theme from Magento 1 to a Magento 2 version involves custom development.
Modules can be migrated to Magento 2 only from a bussies perspective. All custom code logic has to be ported.