Hello aurea196,
You can manually back up your iPhone via iCloud using the steps below:
1. Go to Settings
> [your name] > iCloud > iCloud Backup.
2. Turn on iCloud Backup.
3. iCloud automatically backs up your iPhone daily when iPhone is connected to power, locked, and connected to Wi-Fi.
4. Note: On models that support 5G, your carrier may give you the option to back up iPhone using your cellular network. Go to Settings > [your name] > iCloud > iCloud Backup, then turn on or off Backup Over Cellular.
5. To perform a manual backup, tap Back Up Now.
You can also find those steps in this resource: Back up iPhone - Apple Support
If you receive a prompt about a lack of storage during that process, that would indicate that the size of your backup is larger than your current iCloud storage plan. In that scenario, you can either purchase more iCloud storage or decrease the size of your next backup per the steps below: If you can't back up to iCloud - Apple Support
1. In iOS or iPadOS, go to Settings > [your name].
2. Tap iCloud. At the top, you can check how much iCloud storage you've used.
3. Tap Manage Account Storage or Manage Storage.
4. Tap Backups, then tap the device you're using. Under Last Backup and Backup Size, you can check Next Backup Size. It may take a few minutes for your device to calculate Next Backup Size.
5. If your backup needs more space than the amount you have available in iCloud, you can:
• Delete information that you don't need to reduce the size of your backup
• Upgrade your iCloud+ plan to get more space
If you are instead receiving intermittent prompts about a lack of space, the internal storage of your iPhone may be full. The internal storage of your iPhone can't be increased, and isn't supplemented by your iCloud storage. The resources below have more details on how to manage the internal storage of your iPhone, and further clarifies the differences between internal and iCloud storage:
How to check the storage on your iPhone and iPad - Apple Support
What's the difference between device storage and iCloud storage? - Apple Support
Should you have any further concerns about reviewing those resources, just let us know. We'll go from there.