Where is my ssid found

“Where” will be different for everyone, as it depends on your internet provider. If you only have cellular internet, then you don’t have an SSID.

The router-modem from your ISP for your home internet will probably have a pre-configured default SSID, either as detailed in the documentation or a separate letter, or using a sticker on the side or bottom of the router-modem. Ask your ISP if unsure. But the SSID name can be changed by you to any name you want (up to 32 bytes/characters). See your router-modem documentation. Ask your ISP if unsure.

You can connect your iPhone to a Wi-Fi network in Settings﹥Wi-Fi﹥. There you can also see to which network (SSID) it is currently connected, if any. The phone will also scan for any visible Wi-Fi networks in the neighborhood. Connect to Wi-Fi on your iPhone, iPad, or iPod touch - Apple Support

I believe, it was stated that it is set in your router. However, what it is is probably more appropriate, it's the network name. You probably know it already and have used it plenty of times.

For example, if you take your laptop to a coffee shop and attempt to connect to the local Wi-Fi network, your screen will display a list of SSIDs

The SSID is the network name for the Wi-Fi network, that all your devices connect to. It will be set in your router.