If it has a decent camera, use it as a dedicated webcam. If the camera is just okay, convert it to a car dash-cam or a home security camera with integrated UPS, storage, and even fallback connectivity via mobile networks. Use it as a dedicated gaming device, or a music player for non-IoT speakers. Convert it to an LTE modem and make it a fallback for your home internet. Run a Monero node on it. Or a Briar mailbox. Host a personal website on it and make it available via DynDNS. Make use of the phone’s sensors, e.g. the light sensor or the microphone for home automation. Connect it to speakers and use it as a Bitcoin price monitor that plays “You Suffer” by Napalm Death every time BTC passes a certain threshold. Or just use it as a digital photo frame on your desk.

Just sell them for whatever and buy proper stuff

You have an iPhone 9? It doesn’t exist, you probably have something else.

iPhone 7 got Linux ported, so yes you can. Try looking for postmarketOS (though there aren’t that many working features, it’s good enough for running servers)