Generally, no. The firmware/software is open source, but it uses LoRa which is proprietary. I think every manufacturer has to pay a license fee to use LoRa. You could probably buy a LoRa transceiver module and a general ESP32 or NRF devboard, connect them somehow, configure the firmware, and get something working though.

Yes, you have many options. It depends on how much "from scratch" you want to go.

The simplest method is to purchase a module with the radio transceiver + microcontroller, flash it, and assemble it. If you don't want any sensors, you can for example purchase from RAK the kit a kit with a 'RAK19003' base board + 'RAK4631' module (nRF52840 micro-controller + SX1262 transciever) . For Canada, you would pick the 900 MHz version that operates in the 915 MHz band. (https://store.rakwireless.com/products/wisblock-meshtastic-starter-kit?variant=43884035113158)

For an enclosure, you can look up 'Project box' or 3D print a case.

If you want to go even more "from scratch", you can buy a module without a micro-controller (Waveshare core1262, Ra-01SH, Wio-SX1262), and interface with these using a micro-controller via SPI. At this layer it starts to become more of a hassle if you want to implement Meshtastic, because you will need to either copy an existing configuration, or modify the firmware so that it matches the way that your electronics are connected.

Then, if you do not want to purchase a module, you would buy the transciever directly (for example, the SX1262), and assemble your own module. You can look up the schematic of the basic modules to get an idea of what this looks like. For example, you can see the Waveshare Core1262 schematic here: https://files.waveshare.com/upload/c/c1/CoreSX1262_Sch.pdf

If you do not want to rely on an already existing LoRa transciever, but instead use a more general radio transciever, that is also possible. But, more expensive, and is unlikely to match performance. This is something that one might want to do if you already have an SDR transciever connected to raspberry pi and want to use it to interface with LoRa (still, it is much easier to connect a LoRa device over USB). I would not recommend building a meshtastic device from more general transcievers.

The other commenter have described the challenge, but I'd like to clarify the terminology, since the distinctions might not be obvious. For tech, we generally speak of the separate qualities of being Free (as in, use it however you want) and Open (aka being open to study, reimplement, and extend). If both qualities are had, then that's called Free And Open.

The most common designation is for software, which if both Free and Open, then that's Free And Open-Source Software (FOSS). Examples include the Linux kernel (GPL license) and FreeBSD in its entirety (MIT license). This means you can remake the software and use it how you like.

For hardware, there's also the equivalent concept of Free And Open, and that means the PCB design can be remade and used for whatever you want. If you wish to use Free And Open hardware for war or for hobby use, that's entirely up to you.

But there's also the realm of silicon, which is the most esoteric and specialized, and there's a lot less Free And Open silicon designs available. For example, the x86 CPU architecture is not Free nor Open. It is patented and its logic is proprietary and trade secrets of Intel, AMD, and Via. They document the behavior of registers, but they never publish the silicon designs so that you could make your own at home.

ARM is slightly different, in that they'll gladly help you build your own ARM silicon (eg Apple Silicon system-on-chips) but you need to pay them a license. So it's not Free nor Open because: 1) you have to pay money, and 2) the plans aren't available for examination until you pay up.

LoRA silicon is more akin to x86, because they just don't publish anything except the register behavior. The license to use the LoRA design is baked into the sale price that the LoRA Alliance charges. And yet still, you at home receive no right to remix or examine that silicon design yourself, unless you do actual reverse engineering. And even then, they have patents.

LoRA is not Open nor Free silicon. And it never claimed to be. Meshtastic and MeshCore use Free and Open hardware and software but that's it. You do not have as many rights to the silicon as you do for the hardware and software.

Does that mean it’s possible to build radio receivers by ordering the parts and assembling them yourself, Ikea style? If yes, then how can I go about doing this?

Depends on what you mean by "build radio receivers"

• The LoRa boards that work with Meshcore/Meshtastic are so cheap and plentiful that folks usually just buy the board and assemble it in the case of their choice with battery, antenna, etc, of their choosing. {Most boards available at retail have factory-installed connectors for easy attachment but in some cases one might need to solder.}
• you might mean "build a LoRa board from components" but a person who could do that probably wouldn't be asking the question <- not trying to be a jerk, can't think of another way to put it pre-coffee.

{edited to reword section about pins}