gos It is not easy to tamper with a phone especially the Pixel line of phones. That being said, unless you have been specifically targeted by an advanced adversary, it is very unlikely your phone was tampered with. Despite this, there are steps you can take to verify if any tampering has occurred.
You can try installing GrapheneOS and then pause boot on first GrapheneOS boot. Check the hash value and compare it with the hash for the image you installed on the GrapheneOS website.
If you can confirm they match, you can resume boot and verify using the Auditor app.
If you want you can install a trustworthy third-party app that can monitor your network connections and check if there are any unusual connections.