It is advised that you utilize the Nextcloud client app from the Nextcloud team.
Apps for desktop can be found here.
iOS and Android also have apps.
The apps operate in generally the same manner. So follow the instructions understanding the different caveats of your device.
Logging In
Logging in can be accomplished via your username and password (with your Two-Factor Authentication if enabled – highly recommended).
On mobile devices, you will be redirected to your phones browser to sign in with a prompt to approve. Likewise for desktop clients.
It is highly recommended that you setup an app password that is specific to this app and device via Personal Settings -> Security -> App Passwords. The password will only need to be entered once so no need to retain or memorize it. If the desktop/mobile app requests a new login, simply create a new app password, removing the old one.
Sync
The app will create a folder on your device to store the data locally. You can specify in the app settings to ask about files over a certain size being synced. Recommend to tailor this setting for your device and network. It might be beneficial for your to interact with a large file through a browser or file type specific app. A large spreadsheet is an example of something might run smoother being edited in browser using OnlyOffice or using a spreadsheet editor that supports that file format. (i.e. OnlyOffice for Android or iOS)