Just a quick post for my own reference later, though hopefully it is useful to others. Many new phones have a super partition and the way they are designed makes it difficult to remount the system partition as read and writable. I didn't come up with this, but it took me hours to find online, so I am posting it here, with full credits to the original author. I particular, they were using this to be able to install a system privileged app, but this works to make it r/w for any purpose.

Here's the code:
\$ adb root

\$ adb shell avbctl disable-verification

Successfully disabled verification. Reboot the device for changes to take effect.

\$ adb disable-verity

using overlayfs
Successfully disabled verity
Now reboot your device for settings to take effect

\$ adb reboot

\$ adb root && adb remount
remount succeeded

\$ adb push ScreenCap.apk /system/app/
ScreenCap.apk: 1 file pushed. 33.1 MB/s (1640812 bytes in 0.047s)

\$ adb reboot

And here's the credit:

https://android.stackexchange.com/questions/220370/how-to-modify-system-priv-app-in-android-10

Originally written by Libin Jose

https://android.stackexchange.com/users/326624/libin-jose

Linux - keep it simple.