computer starts with grub after installation - what should I do

Normally GRUB is not displayed, but it will be displayed for several seconds then move on to boot the PC if there are other kernels or operating systems on that PC.
Did you try to press enter?
Post a picture here on this forum if it does not go any further.

Yeah, that.
In BIOS, find and disable the UEFI, set the boot options to Legacy, and disable "secure boot".
In some cases this is called CMS, so try with it disabled and enabled.
The bad thing is you might have to install again after changing those settings.
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When you boot from USB, press F9 or F12 or whatever your PC has to select a bootable device, then choose the USB in Legacy mode, not in UEFI.

If it works, leave it as is.
I am sure Andy did a lot of testing with UEFI and it works in most cases with it, just some BIOS implementations are very, very wrong. I have a small laptop/tablet HP X2 Elite that refuses to boot anything over Mint19.3, i have tried everything possible under the sun, even did separate partitioning for everything, EFI included, it just gets to a blank screen.
Everything boots and works properly from USB.
But i like the damn thing, it can run Mesa boards all day long through a USB to Ethernet dongle!

You are welcomed, always.
There are plenty of simulated configs included, so you can start and use them with no hardware attached.
Also open the configuration files with a text editor and look at how things go there, this was the most important thing for me when i used LinuxCNC for the first time, editing a simple text file to modify everything!