JacksWendel Do the various linux-phones also come with closedsource firmware?
By no means have I done an exhaustive inventory, so maybe there is some "Linux phone" that has 100% open-source firmware. But some of them have closed-source bootloaders, a firmware binary blob for the cellular modem, a firmware binary blob for Wi-Fi and/or Bluetooth, etc. Though I don't want to get into putting down any particular device or project, the existence of closed-source firmware can be verified by simple web searches.
For example, imagine there is a phone called the EchidnaPhone (I chose that name because I think there isn't one). Try a web search for echidnaphone binary blob and see what turns up, except swapping "echidnaphone" for the name of some "Linux phone" that you are interested in reading up on. And/or echidnaphone baseband source code.
Often there is some source code for some firmware. But often some critical part, often the cellular modem, is running closed-source code. Often the cellular modem is isolated in some way from the main system, which is also true on Pixel devices, though the isolation on Pixel devices arguably depends more on the Tensor SoC. But that doesn't address some concerns people express about closed-source cellular baseband firmware, such as "Is the closed-source cellular baseband firmware transmitting surprising information about my activity behind my back?".
Meanwhile, as far as I know, there isn't a "Linux phone" that has solid hardware security, such as Apple's secure element or Google's Titan series. Now that MTE is starting to be more mainstream on ARM, over time some of those devices may acquire MTE, which Pixels have had since the Pixel 8 in October 2023.
So the short answer to your question is that, as far as I know, "Linux phones" have some closed-source firmware plus some degree of mitigation of that (the same is true of Pixels, at a high level). However, as far as I know, the "Linux phones" are generally years behind on some important security features.