Open Settings > Import & Export and pick your source. There are dedicated importers for Apple Notes, Obsidian, Standard Notes, Google Keep, Simplenote, Samsung Notes, and Bitwarden (logins, cards, and secure notes land in the Vault), plus a generic Markdown importer for any folder of .md files.
Imports run entirely on your device: your exported files are parsed, encrypted, and stored locally, nothing is uploaded for processing. Each imported note is tagged with its source so you can review the batch afterwards, and Google Keep checklists even convert to native task lists.