Why are my Apple devices too linked

First, perhaps understand the Apple Support Community is a user-to-user technical forum - and not a chat-room where you will receive an immediate response. Here you post a question - and then wait for a reply. If you are lucky, a knowledgeable contributor will see your post and reply with [hopefully] useful information or guidance.

Contributors here are all end-users, just like you. Other than the site Moderators, Apple neither monitor nor participate here. As such, if you expect to receive a reply from Apple through these forums, you will be disappointed.

Returning to your question, your iPad and iPhone are tightly integrated - and use multiple Apple services to synchronise and exchange data. When correctly configured, an iPad can access Cellular and WiFi Calling features of your iPhone using the FaceTime App...

When used with an iPhone, the capabilities of your iPad significantly broaden to include access to Cellular Voice, WiFi Calling and SMS/MMS Messaging services of the associated iPhone - relayed to the iPad over WiFi using Apple’s Continuity features. For the most part, an iPad using Continuity is able to access Cellular calling features, of the iPhone, when both the iPad an iPhone are in close physical proximity and are connected to the same WiFi network - but this is not the end of the story.

If your both your Cellular Carrier and your calling plan support WiFi Calling, your iPad can make and receive calls independently of the iPhone - even when the iPhone is switched off, elsewhere, or using a different WiFi network. Not all carriers support full WiFi calling.

Additional information about Continuity and the relevant Cellular services can be found here:

Continuity

Work across all of your devices seamlessly – Apple Support

SMS/MMS Messaging

Forward SMS/MMS text messages from your iPhone to your Mac or iPad - Apple Support

WiFi Calling

Make a call with Wi-Fi Calling – Apple Support

Making and receiving calls on iPad

Make and receive phone calls on iPad - Apple Support

For iPad, in addition to regular FaceTime calls, the FaceTime App provides access to Cellular Voice Calling features of your iPhone - whilst the Messages App supports SMS/MMS messaging services.