Well you are limited through the "Glücksspielstaatsvertrag" (it is a Law despite the cute name).
You are allowed to play only on the sites on the
whitelist, these are GG-Poker, PokerStars, 888-Poker, PartyPoker, bwin and bpremium (PartyPoker, bwin, bpremium are Entain skins). Most other sites that are illegal to play on from Germany do GEO-Blocking(WPN, ACR, ....).
Remember:
§285 STGB Teilnahme an unerlaubtem Glücksspiel.
For CC Freerolls use the referral links fron Cardschat to register there.
Most freerolls has
888 and best CardsChat coverage for German players but the traffic is very limited.
PartyPoker and only PartyPoker have limited Freerolls and some good CC ones if you registered through the referral link from CC, traffic is also not so great.
GG has some Freerolls also some CC ones but limited, but the site with the best traffic.
The former leading room
PokerStars has no Freerolls and nothing for German players (except some centrolls) that may encourage a poker player from Germany to play there.
This is the perspective for Freeroll players.
The rest is up to your decission, software, stability game choice is another topic.