Is there a way to quickly scan all my images in a new pc?

The images were copied, and just to be clear, did you also copy the Lightroom Classic catalog folders from the old PC? It almost sounds like you started over on the new PC, with the existing images but starting a new catalog and importing everything fresh. Is that what happened?

If a Lightroom Classic catalog folder was copied from the old PC, I would think the previews that are normally inside that folder would come along of course, and should not have to be regenerated. Unless you are saying you did bring the old catalog folder(s) but are now looking at images you have not worked with in some time, so previews might not exist for them and need to be built.

Regardless, there are several options here.

You can make Lightroom Classic pre-build previews for everything. In the Library module, in the Catalog panel, select All Photographs, which displays all photos in the catalog. Then choose Edit > Select All, then choose Library > Previews and on the Previews submenu, choose either Build Standard-Sized Previews or Build 1:1 Previews. This will pre-build previews for all of those selected images.

If that Ryzen has lots of cores, that operation should go faster if you first go into Lightroom Classic Preferences, and in the Performance tab, make sure Generate Previews in Parallel is enabled.

For the future, another option you may want to enable is in the Preferences/General tab: Replace Embedded Previews with Standard Previews at Idle Time. This will let it pre-build previews for any raw files that don’t yet have one, when it thinks you’ve walked away from the computer for a while.

For raw file imports, in the Import dialog box I like to use the Embedded & Sidecar option for Build Previews because then Lightroom Classic will not try to build previews before showing you the images; it will use the preview that the camera already embedded with each raw file. That means I can look at them right away. However, as soon any Lightroom Classic option is applied, an image’s preview must be regenerated using the Lightroom Classic raw processing engine.

After Selecting all the images, you can try menu "Library -> Previews" and select which preview level you want.

I bought a new pc and I translated everything to it, using the backups and the images. around 20k images, but the problem is each time I want to work with these old images images for first time these have to load everything, the presets and the images wich makes working with my images hard and tedious, I have to be scrolling my library and waiting till these load fully after that they are back to normal. but is there a way to make lightroom scan all the images at once? If not it really should be, becuase this makes working with lightroom anoying if update your pc. Lightroom scan new images and learn about them when you see them but I wish you could make lightroom scan all library to avoid this.

Are you talking about not being able to see the thumbnails in the Library Module the first time you launch Lightroom Classic on your new PC? Are you talking about Lightroom Classic?

Lightroom Classic has to generate thumbnails for 20K images, this will take some time. No way around it (unbless you copy the Preview cache from old PC to new PC, which itself will take a long time).