I have no idea what regtest is or how to do what you said there.
There are many networks you can pick:
Chain selection options:
-chain=<chain>
Use the chain <chain> (default: main). Allowed values: main, test,
testnet4, signet, regtest
Again, read help carefully, it has a lot of information. If you don't want to download any blockchain, then regtest is just for you, because then, you can run everything locally.
My question is, this is supposed to not download blocks before the given hash
No, everything is always downloaded. If you assume, that blocks are valid, you simply don't check signatures. But the SHA-256 of the block is still checked, to make sure, that you received, what you requested. And your node still downloads everything, because the UTXO database has to be made out of something: if you won't download old blocks, then how do you want to get information about old, unspent coins?
if I use 1, that I assume is a reasonable one since the devs selected it
If you use "1", then it is invalid choice, because block hashes have 256 bits. Use "0", don't use it at all, or put the block hash you want. But don't use "1".
I was wondering, is this info possible to be displayed in the full node console command GUI?
Of course. But more than that: if you have a full node executable, with the GUI, then you can also get it, just by adding "--help".
So why does the assumevalid value go further back in time than the mainnet value?
Because it is manually updated every sometimes. There is no rule, when it should be updated. You can replace it with your own block, mined a day ago, if you want to.
And then once it's fully synced, prune=1000 enters in action and deletes all the blocks except the last 1GB?
Things are removed automatically, as they are checked and downloaded. But: the size of the chain is not all: there is also the size of the UTXO database, and it can take potentially unlimited space, because the number of UTXOs is not restricted by consensus rules. Usually it won't take more than a few GBs, but still: it could grow in the future.