Salesforce protects our data. But does it protect the data from Salesforce itself ? Is my data always visible/accessible to Salesforce?
The data you store in any cloud service is, ultimately, accessible to that cloud service, saving only the circumstance that the data is encrypted on your local machine prior to transmission to the cloud and you and only you hold the encryption key - i.e., it's never transmitted to the service itself, and all data is decrypted only on your local machine.
There are relatively few cloud services that meet that definition, because it imposes very substantial limitations upon what the service can do. Most services that fall into this category are things like password managers and bulk data stores. Even in those cases, of course, you're vulnerable to manipulation of the client software (supply chain attacks, in essence), suborning of your local machine, and all of the other vulnerabilities that come with handling key storage - right down to the subpoena or the broken fingers, depending on what your threat model actually looks like.
Salesforce shifts a lot of those concerns from you to them. They provide a highly sophisticated security architecture, protected data centers, and a staff of trained security engineers. There are various tools you can use to protect your Salesforce data against different types of threats. The highly sophisticated platform security mechanisms offer protection against your own users, and help prevent the creation of vulnerabilities like CSRF and SOQL injections that your developers might create inadvertently. If you purchase the Shield Platform Encryption add-on, you can even encrypt your data (well, much of your data) while at rest on Salesforce's servers, to meet your compliance needs. Even with Shield, though, the keys reside with Salesforce and in theory could be accessed by the company.
The landscape of data security is very, very complex. In the context of a major enterprise cloud service like Salesforce, it's generally more useful to ask yourself, and your vendors, questions like "What safeguards do you put in place to protect my data from internal threats?" or "Who at $VENDOR able to access my keys?", or "What verification challenges do you require before you discuss my account with a caller?" rather than "Is it possible for you to access my data?"
The answer to that last question is almost always "yes", given enough time, legal compulsion, or money.
Your chats are visible to Facebook, WhatsApp so does your emails and search history to Google. In the end, everything is saved on hard drive(Cloud also have harddrive) so it can always be accessed... Same happened to bitcoin hack couple of months back when someone got access to the private key of bitcoin enterprise.
Coming to your question, Can Salesforce can access your data? Ofc their data engineers and bots can and thats how the Einstien analytics reads analyzes and gives your predection. No AI can be achived without data content and type study. So yes salesforce is reading and accessing your data. But it's for improving the salesforce AI and not for selling to competeitor.
Should you be worried? No,
When you read security document for salesforce they have written,
Your data is secure with salesforce.com. Your data will be completely inaccessible to your competitors.
Trust is the number one priority of Salesforce, if they break it for one customer, they would just stop getting business from others. So they would never dream to do that.
Can any salesforce employee see my data? No, Only some set of data engineers can access your data as they are at data centres where it is stored, but every access is logged.
Also, if you Grant login access to Salesforce Support agent, they can also see and view your data, its also monitored and recorded.