Let us say I have a HD wallet. Can I get it’s seed phrase from the private key of one of its wallets?
No, the mathematical processes are one-way processes. See BIP 39 and PBKDF2
Also are seed phrases just easy to remember private keys?
Seed phrases are not intended to be memorised. They are intended to be written down and stored somewhere safe. For example you might use letter punches to stamp the phrase into stainless steel sheet and store the sheet in a safety deposit box.
from the seed phrase private key you can get a public key?
Yes. See BIP 32
if I send cryptos to this public key in which HD wallet is it going to appear in?
Money will appear in, and be spendable from, every appropriately-configured wallet created using that seed-phrase.
If you are thinking of each key pair derived from the master node as a wallet in its own right (which I think is a misleading way to think of things, or at least unhelpfully ambiguous) then the money is only associated with one of the key-pairs in one of the "wallets" derived from the "master node". But that "wallet" (depth 1 derivation) can exist in many wallets (multiple wallet software installations on one or many computers)