SegWit introduces witness data for spending validation, separated from the serialization used for txid. It is a consensus change deployed as a soft fork, not the removal of signatures from validation.
txid excludes witness; wtxid includes the new witness serialization. Changing signature data for the relevant SegWit inputs therefore does not change txid; this does not prohibit every transaction change. A coinbase witness commitment binds witness data into the block, where validating nodes still check it. [BIP 141: Segregated Witness]
BIP 141 defines weight as three times base size plus total size, with a 4,000,000 weight-unit block limit. A base byte costs four units; an additional witness byte costs one. Virtual size is weight divided by four, rounded up; actual byte count depends on block composition. [BIP 141: Segregated Witness]
P2WPKH and P2WSH use witness version 0, nested in P2SH or native. A bc1 prefix alone does not specify every spending condition: Taproot uses witness version 1. SegWit guarantees neither anonymity nor always lower absolute fees; transaction structure and the selected rate matter. [BIP 141: Segregated Witness] [BIP 341: witness version 1]
For the clearest picture, read this entry together with Transaction, Transaction fees, Lightning Network, Taproot. The reverse links also lead from Transaction, Transaction fees, Taproot, Lightning Network.