The way ETFs are usually structured is by setting up a trust. This trust buys and sells the underlying securities from the open market, as per its mandate. So for instance, NIFTYBEES has the mandate of holding companies in the NIFTY, as per their weightage in the index.
When you buy a unit of an ETF that is listed on the market, you are essentially buying a share of ownership of all the assets held by this trust. The market price of the ETF at any given time might be different from the NAV (net asset value) that is declared every day.
The units of the ETFs are usually listed at the time the fund is launched, at the time of NFO. There is no separate “IPO” step that is required. For example, the NFO for Motilal Oswal’s 5 year G-SEC ETF is happening now, the units of the ETF will be listed on the markets after the NFO process is complete.
P.S- You’re not required to pay 13000 in each Nifty ETF, merely because the NIFTY is at 13000 today. Each ETF has a different ticket size, based on how many units the trust’s assets are divided into. For example, one unit of NIFTYBEES is benchmarked to 1/100th of the value of the NIFTY, so trades around Rs 130 per unit.