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The OS looks for updates from the manufacturer's site for a newer version. When you click download on your phone it download the latest update usually as "update.zip".
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The updates are still required to be done by the OEM. And if you try flashing a GSI yourself, it will either not boot or be ridden with issues.
The underlying vendor partition also needs an update along with new and updated drivers and HALs from the component vendors to make the hardware work properly with the new OS version.
Hence, it takes time to merge, ...
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Try using do not disturb, that usually helps by going to Settings > Sounds and notifications and check that the following are selected to how you want. There are two options:
Volume: To adjust the volume level for music and videos, system sounds, call ringtones, and notifications.
Do not disturb: Mute incoming call and notification sounds, except for ...
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