Will my existing Magento extensions work with Hyvä?

Some will, some won’t. Hyvä-Compatibility-Module is required for every Luma extension to render frontend HTML — most popular extensions (Mageplaza, Amasty, etc.) ship Hyvä compat modules officially or via the community. The pre-project audit catalogues every extension on your store and flags which need (a) the vendor’s compat module, (b) a custom rebuild, or (c) replacement with a Hyvä-native alternative. You see the report before any work starts.