Illegal instruction is what you get if you try to execute a binary compiled for, eg., x86(-64), so almost certainly that's what's happened.
Which implies the miniconda installer is to blame. To confirm the premise, you need to find the binaries that
conda are trying to execute --
conda may be itself an executable, I'm not familiar with it. You can then examine these with
file; here's an example of what I mean using a relatively stock RpiOS 64-bit:
> file (which pip)
/usr/bin/pip: Python script, ASCII text executable
This is how we know
pip is actually a python script and not a binary executable file. The first thing in the script is of course
> file (which python)
/usr/bin/python: symbolic link to python2
> file (which python2)
/usr/bin/python2: symbolic link to python2.7
> file (which python2.7)
/usr/bin/python2.7: ELF 64-bit LSB pie executable, ARM aarch64, version 1 (SYSV), dynamically linked ...
Here I've followed a set of symlinks to the actual binary. Here's the same thing from an x86-64 desktop python3:
> file (which python3.9)
/usr/bin/python3.9: ELF 64-bit LSB pie executable, x86-64, version 1 (SYSV), dynamically linked ...
Notice the platform architecture is shown: "ARM aarch64" and "x86-64".
If what you get isn't the former, that's what's wrong. Otherwise, try:
> sudo apt install strace
[ ... ]
> strace pip
strace produces copious output detailing the system calls that are being made. Paste the last 25 lines into your question.