Instantly doesn't actually do any of the sending - we send the email information to the email provider of the sending accounts you've connected, which then sends your emails.
So you will not need to add any SPF/DKIM records for Instantly, but you will need to add them for your ESP. Check your ESPs documentation and how to obtain these records. You can check our help center for instructions for the most popular ESPs: https://help.instantly.ai/en/collections/3316199-setup
Our servers (IPs) send information like when the email needs to be sent, what it needs to look like (including styling, variables, spintax) + things like the open tracking code using your custom tracking subdomain. This information is sent to your connected sending account's mail servers, which then do the actual email sending.
We keep rotating our IPs (that send over this information) every few hours to ensure that the accounts themselves don't run into any reputation issues.
You need to get email inboxes from email providers such as GSuite, Outlook, and others. And that is also why users need to warm up their specific domains / sending accounts for 3-4 weeks minimum.