Sats/byte means satoshis per byte and it was the canonical unit for writing transaction fees for a long time. When Segwit was deployed, its transactions used a new fee called sats/vbyte. This answer explains how to convert between bytes and vbytes.
Bitcoin Core uses BTC/kB, also called bitcoins per kilobyte, as a unit in some places. However, some parameters such as minrelaytxfee internally store the fee in sats/kB. To get the sats/kB fee rate, you can just divide the sats/byte value by 1000. One Kilobyte has 1000 bytes (there are other units that measure bytes in powers of 1024, but they are not used by Bitcoin clients).
Similarly, the above procedure can be used to convert sats/vbyte, also known as sats per virtual byte, to sats/vkB – sats per virtual kilobyte.
In order to get the rate in BTC/kB (BTC per kilobyte) from sats/byte, you need to divide the sats/byte value by 100000 (one hundred thousand), because one sat/byte is equal to 0.00001 BTC/kB according to this reddit post.
