Whenever iOS update is available, it rolls on all devices at the same time but Android updates are little different than rest of operating system updates. Google devices will get the updates in the first phase and rest of the devices from other manufacturers can take weeks or months to receive an update.
Most third party phones are running a slightly different versions of Android with some tweaks. Whenever an updates arrive, manufactures have to make some tweaks to adjust the new version of the OS before they have to push the updates. For every phone, date of update can be different. For some android users, the wait is only a week or two, while others may have to wait for a longer time to see the updates in their phone.
If you have a Google Pixel or an Essential then you can have Android P now. Both pixel and essential have the full version of the operating system available now. Nokia has already started to roll in the updates. The Nokia 6.1 and 6.1 Plus have already received the update this month. Sony plans on rolling out the update. Sony seem to have update in November for its premium flagship smartphones. The Nokia 8 and 8 Siroco will get the updates around in November. Motorola will also update its premium flag ship operating system only. The OpenPlus 5/5T and 3/3T will be receiving the update in the upcoming months. Samsung hasn’t confirmed yet about the expected updates, but rumors suggest early 2019.