System updating all the time, still on 1803 build

Hi, please have a look at your Update History in Settings. Is there any indication of failures?

What is your exact system build? Windows key + r, winver.
The latest (non-insider) build for 1803 appears to be June 26, 2019 - KB4509478 (OS Build 17134.860)

If your 1803 is up to date, then when a feature upgrade is offered, you should see a 'Download and install' option. That gives you control of if and when that is applied.

That you say 'My system downloaded the update and installed it yesterday (build 1903) ' as if this happened without you specifically approving it using the new 'Download and install' option suggests it is not up to date.

The old pattern, where a feature upgrade failed, was for that to repeat and repeat. (Bad thinking by MS).

You say:

My system downloaded the update and installed it yesterday (build 1903) but I am still stuck on 1803 build.

then you say:

why I am still missing Clipboard option?

I can't understand this- sorry.

It seems like I am all up to date (mentioned build is the same for me).

So, to be clear please, what is your current Windows build?

Windows key + r, winver

From what you say, either you're on 1803 and repeatedly failing to upgrade to 1903, or you have indeed upgraded to 1903 but don't have this feature- which you wouldn't if the upgrade had failed. Totally unclear at this point...

Glad you made it to 1903. Remember to consolidate by creating a new base disk image with e.g. Macrium Reflect (free) + external storage for image files.. and then routinely update your disk image so you've an option to restore to a previous good state.