I wanted to flash boot.img from magisk but my usb cable killed my pc. Is there any other way to root i/ And if there is any way to do something with that pc? Iys not charging after plugging usb it was okay for 10 seconds and then died.
If you have only 1 USB port in the PC, try a PCIe USB card.
As for rooting with Magisk without a PC, unless you can find a port of TWRP for it, I doubt it. (There's no official TWRP for the A20, and flashing Magisk requires TWRP as far as I can see from everything TopJohnWu has said. You can try the one at https://mega.nz/folder/LTZlUKyI#oxxVc5zj8tP4bNQBIo8AlA - no guarantees, and no saying how long it's going to be up there.)
If you get TWRP running, Install (in TWRP) the latest Magisk (not the manager, although you'll need that installed too - as an app) from TWRP.
Running the manager after that should tell you that Magisk is installed.
On second consideration (after thinking for a few minutes), you'll probably need a PC to install TWRP. So maybe use someone else's PC to install Magisk? (Then you can just patch a boot.img file [that you can extract from a BL tar] and flash it with Odin.)