To view the code:
On the Contract tab of a given contract on BscScan, there is a button that says "Decompile ByteCode". Click that and you'll be taken to a new page called "EVM bytecode decompiler". Then click the button on that page that also says "Decompile ByteCode". The tool will provide you with the decompilation result. Some code will be perfectly decompiled because the explorer has seen it before, while other code will be somewhat obfuscated.
To interact with the code:
This is a bit tricky if the author hasn't published the code. You'll need to send a transaction to the contract and pack the bytes in such a way that the contract understands them and processes them correctly. There may be a tool that helps with this but I have not yet come across it. You may just have to byte pack yourself and send a transaction to the contract using a script.