They will create the code into new release for example new Bitcoin Knots version.
We will run the new version prior the start of the soft fork. Majority of nodes will need to run it to be successful. Hopefully majority of miners will join too.
It will be encoded in it that it kicks in at some time automatically.
If successful the new blocks will fullow the new consensus rules which essentially limit the space in blocks to only monetary use of the protocl.
We are talking Bytes and up to hundred of Bytes but not 100 000 Bytes blobs for arbitrary data which poses huge risk of spam and csam.
Here a bit more info.