Pension for retirement & online brokers

Hello everyone. A newbie here.

I want to start saving for my retirement. I’ve started the research and I’m curious about pensions through online brokers (like DeGiro).

However, I have several doubts:

• How reliable are online brokers for the long run? (20+ years)

• Is it possible to withdraw the pension any time without penalties?

• How it works if I move my residence from one country to another? I live in The Netherlands at the moment but might move to Spain in a few years. Is there any online broker that works through EU countries?

• Besides DeGiro, could you recommend me other options specifically for pensions?

I would appreciate any thoughts and advise in any of the questions above! Thank you so much for your time.