sole trader and one man company

main difference is sole trader has unlimited liability and one person company has limited liability

both the orgn are their or sole trader is replaced by one person company

Both are there.
Don't worry.
Govt. has introduced a new concept of OPC since co.act2013, as per which an individual who is citizen & resident of India can become the member of OPC.
the main difference btw.sole trader & OPC are as:--
⏺️OPC (one person co.) have separate legal entity;
🔽while in case of sole trader it is not.

⏺️ liability of the member of OPC is limited;
🔽 while liability of sole trader is unlimited.

⏺️OPC is governed by Co.Act 2013;
🔽while no act for sole trader.