If you provide such minimal information, we can't figure out what's going on. We are not overlords that know everything that happens on your computer. We don't even know whether you have run some other commands, and it looks like you just don't do anything apart from the commands we suggest.
Have you read the docs? What steps have you followed? Have you tried removing the core folder where Volto core is cloned? Have you tried going to the core folder and check with the git command which is the downloaded version? Where are you seeing that the Volto version hasn't changed? What is the output of the make install command after changing the Volto version in mrs.developer.json?
If you really want help, provide the most information you can, otherwise no one will be able to help and we will stop answering, and then you will be mad because nobody wants to help me.
Cloning core from https://github.com/plone/volto.git...
✓ cloned core at core
✓ update core to tag 18.21.0
Scope: all 20 workspace projects
Lockfile is up to date, resolution step is skipped
Packages: +2978
And the display after finish is still
It mean still 18.21.0, thats why I can ask help again.
@mactrash mrs.developer.json is for development. If you clone "core" ideally with the provided volto version that means its going to clone it for your dev setup wherever you want. Not as a library, that would reside in your node_modules. (the above step also creates a symlink to node_modules @plone/volto, but I'm not sure if that's still the case. )
Anyway, You can do 3 things:
Check if you have @plone/volto pinned to something bad in resolutions field. However that's pretty unlikely.
Do a pnpm why @plone/volto or better pnpm why @plone/volto@18.21.0 and inspect the result.