How do I make a mac have a faster boot time?
Here, I assume your iMac has a 21.5&quot; screen and was original configured with a internal 1 TB HDD. In this case, your model does not have the PCIe connector for an internal blade SSD. Therefore, you could only replace the internal SATA HDD with an internal SATA SSD. Note, this SATA connector operates at a maximum of 6 Gb/s. So, instead of replacing the internal drive, you would probably be better off with an external 10 Gb/s USB or 40 Gb/s Thunderbolt 3 drive. Also, an external drive would be easier to try and if purchased properly, the drive could be returned if the performance is not satisfactory.
Your model Mac is not designed to be taken apart. Although, opening the Mac to replace the SATA HDD with SATA SSD would be possible. When I purchased my 2013 iMac, I had Micro Center open the tapped edge Mac and upgrade to 16 GB of RAM. Like your Mac, the internal drive is a 1 TB HDD. Upgrading to Catalina made this Mac unusable. APFS was not made for HDD. I added an external USB Samsung T7 SSD as the boot drive. Even though this Mac only has a 5 Gb/s USB type A port, the performance improved dramatically. This includes the boot time. Your Mac has 10 Gb/s USB type C ports, so I would expect even better performance. Evidently, Macs preform TRIM operations when booting, which can increase boot times. However, the Samsung T7 does not support TRIM, so this will not be a factor. There are reports stating the lack of TRIM will eventually slow down a SSD, but I have not noticed any slow down. I theorize that even at the drive's slowest speed, the drive is still operating faster than the 5 Gb/s USB interface. For reference purposes, below is the output from the System Information application. Your Mac would report a speed of up to 10 Gb/s.

Another possibility is to add a external Thunderbolt 3 Samsung X5 SSD as the boot drive. These drives are significantly more expansive that the Samsung T7, but the interface is 40 Gb/s. I have one of these drives connected to a 2018 Mac mini. The Mac reports that TRIM is supported and enabled when the Samsung X5 is connected. I have booted Mojave from the PCIe-based internal SSD storage and the external Samsung X5 SSD. I can report that I do not really notice any difference in performance. However, the specifications state the Samsung X5 should have a slightly slower transfer rate. For reference purposes, below is the output from the System Information application.

I should also note that the external Samsung X5 is also reported to be a NVMExpress device, just like the PCIe-based internal SSD storage. (Boot Camp was added manually. You can not use the Boot Camp Assistant to install Windows on external drives.) Note that the Thunderbolt drive is disk0 even though the Mac is current booted to Mojave from the internal drive.

References:
How do you upgrade the hard drive or SSD in the &quot;Tapered Edge&quot; Aluminum iMac models? What type of storage do they support? Is it even possible to upgrade the storage?