European developer here having worked across 3 countries in Europe and also involved in hiring/recruiting.Here's my advice:

Sending your application
If (bigger) startup has an internal recruitment/HR who will review you CV first. As sad as it may sound, make sure their 'keywords' match yours in the CV. Except for the internal recruiters with a technical background (close to none), they will just go through the list of keywords needed for the role. It's more or less a bulletproof approach to pass the first filter which is essential if you even want to be considered. It's a pity that persons who have almost zero knowledge about the technologies we use have so much impact and power on deciding whether or not you'll be good based on their criteria (again; keywords).

Most internal recruiters don't care about a cover letter at all, with a big part of them not even reading it. Therefore, keep it very short and rather invest time in a good CV.

You may need to adapt your CV specific to each role specifically (focussing on the role requirements), so be ready to have a CV that can easily be adapted to a specific role.

CV
2 pages is ideal (especially if you've held >2 roles), first block is your personal information, followed by your work experience (latest position first). Make some bullet points of what your tasks where at each role. Bullet points are always a win in a CV, always have readability in mind, no HR/Hiring Manager wants to get a long wall of text CV after having gone through 20 previous one's.

After that, you can always put your Github or SO accounts (more likely if startup), but I would rather talk about this with the Hiring Manager during the phone interview, and if they are curious ask them if you can send your account via email, this also gives you a good opportunity to add some extra details about you which can make you stand out better and get you 'remembered' longer and more positively if you are in their inbox. You would be surprised how many (older) technical managers still don't know SO or Github.

Then comes your education, just very short, only mention colleges/uni if so.

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Always print your CV and look at it skeptically and ask yourself "Would I like getting a CV like this?"

My experience with places I've worked
London
Quite hard to even get a response, I think companies in London have an easy time hiring with many good candidates. Also no relocation packages or anything like that.

Holland
Many tech jobs available and good chances to get hired when not native. Dutch people are quite keen on international employees.

Spain
Good supply/demand ratio in favor of demand. From what I've seen tech companies in Spain have hard time hiring quality employees. Also pretty international (Barcelona, Madrid, Malaga).

Other
I understand the question is more about the first phase of applying, but when you secure an interview at a company, learn as much as you can about them, you might be asked for it.

Basic flow of an interview in Europe is: Company introduces itself -> you introduce yourself -> Q&A specific for the role -> question round where you are free to ask questions. When they ask you to ask questions, do so! Company owners/Hiring Managers/HR love to talk about their company and it's a good way to find out about the culture, and it makes you interested in the company which is an advantage. Startups usually don't give you any tests or tasks during the interview, they usually stick to asking technical questions. Last but not least, if you are very interested in working at a company and have a interview at their office, make a short presentation. Especially for technical roles this is highly appreciated. Put some effort in this (5-15hours) and use whatever you can or is related to your role. The presentation should match the "you introduce yourself " part of the interview and go a bit deeper into your knowledge. Present it with confidence. When they like it, you will see the interview will get a lot more relaxed and if you've given a good insight in your knowledge/projects you've worked on a lot of the technical questions will already be answered or skipped.

The presentation part was twice successful for the 2 roles I ever applied for. I've also advised it to 3 friends of which 2 got hired, of which both the Hiring Manager said they were impressed by the presentation.