Raul Garcia has retired and Muniain also left the club after 15 years (somehow he's only 31, was really around so long I'd expect him to be in his mid-thirties). Both are club legends but neither played much last season so them leaving shouldn't impact the team much. At least not on the pitch, they will surely miss their leadership and experience but Bilbao has a very strong culture and group of players who have played together for years so that shouldn't be a problem.
Main question for next season is if Nico Williams will stay. They've signed Braga's winger Alvaro Djalo for around 15m and he'd be a good replacement for Nico if/when he eventually leaves but if he stays for another year then Djalo will be a very good added value to the team.
Bilbao reportedly also have a deal with former Sociedad RB Andoni Gorosabel who played last season with Alaves and will join as a free agent to compete with aging De Marcos.
They also want to bring back Laporte from Al Nassr and are interested in midfielder Ivan Martin who had a breakthrough season in Girona.
If they will keep Nico and sign these three players in addition to Djalo then I would consider Athletic favorites to finish 4th next season.
I don't expect Girona can repeat this season. They'll lose Aleix Garcia, Savio, Couto? and maybe Dovbyk and while I don't doubt they'll replace them with quality players (CFG will do their best to help them) I believe main problem will be they're not used to play in Europe so their squad depth will be challenged.