Where is BOOTX64.cfg?

There is no BOOTX64.cfg.

There is only BOOTX64.EFI

And it’s uneditable and it can’t even open properly. Bunch of symbols there.

Used Rufus, Windows 10.

P.S. Goes blackscreen after Grub too. Can’t go to legacy mode cause Intel doesn’t support it (comp without separate videocard). Now i need to buy a videocard to be able to turn on legacy mode. Ridiculous.

You are right - there is however grub.cfg - make the changes there.