It looks like buying before listing, but the actual process has several stages.
A company selling shares publicly for the first time is making an offering. It appoints underwriters and runs a bookbuilding process asking institutional investors how much they would buy and at what price. The offer price is set from those results, and retail subscription follows. By the time individuals apply, the price is already fixed.
Receiving everything you applied for is rare. Where demand is high, allocation is made proportionally, often combined with distributing a minimum quantity evenly. Committing a large amount can therefore return fewer shares than expected, and the unused portion of the deposit is refunded after allocation. Whether you may apply through several brokers at once varies by offering.
Offer prices are generally calculated by comparing metrics with similar listed companies, then applying a discount. Which companies were chosen for comparison and which metrics were used are stated in the prospectus. Reading the assumptions behind the price helps more than reading the price alone.
Some existing shareholders and institutions receive shares on a commitment not to sell for a set period. When that ends, the supply available to the market increases. The dates and amounts are disclosed and matter for understanding supply in the period after listing.
There is an impression that IPOs rise on listing day, but that is not always so, and opening below the offer price is not unusual. It varies with market conditions and the bookbuilding result. This explains the process and terminology; it neither recommends nor discourages subscribing to anything.
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