SIGHASH_ALL commits to all inputs and outputs and is what wallets use almost always; NONE leaves outputs open, SINGLE commits only to the output with the same index, and ANYONECANPAY lets others add inputs (crowdfunding-style). Taproot's SIGHASH_DEFAULT (0x00) behaves like ALL and saves the byte. BIP-143 (SegWit v0) and BIP-341 (Taproot) define how the digest is built, including input amounts, which lets hardware wallets verify fees.
Unusual flags in a PSBT are a red flag: a signer tricked into SIGHASH_NONE or SINGLE lets someone else redirect funds. Signers should refuse non-default flags unless the protocol needs them.