How to check RAM in iPhone

There's really no way. Each iPhone model has a set amount of RAM that's the same regardless of the storage capacity. Apple doesn't publish the amount of RAM in their technical specifications, but many sites have that information available.

On which iPhone model and again, why?

Ajman2022 wrote:

To know the RAM of the iPhone (4GB or 8 GB)

There are myriad iPhone models, most with differing internal RAM memory.

Without the specific iPhone model, you will get no answer here.

You can search for websites with the technical specs for a particular iPhone and its RAM using the Google or DuckDuckGo search engines yourself, or can use an app such as the (free) MacTracker reference app:

https://apps.apple.com/us/app/mactracker/id311421597

Put differently, we’re looking up these answers using the same resources already available to you. But to answer your question (here), you need to tell us which specific model of iPhone. Or you will want to search for the specific iPhone model using available tools.

Ajman2022 wrote:

To know the RAM of the iPhone (4GB or 8 GB)

No iPhone model to date has more than 6 GB. There are a lot of different sites that do publish the amount of RAM for each model. But it's fixed for any particular iPhone model regardless of the amount of storage.

For instance, the Wikipedia entry on the iPhone 13 Pro notes that it has 6 GB of LPDDRX4 RAM.

https://en.wikipedia.org/wiki/IPhone_13_Pro

Apple doesn't list it at all in any public specifications.

iPhone 13 Pro and 13 Pro Max - Technical Specifications - Apple

What do you mean by check it? IOS controls the RAM in the iPhone, there is no user options.