Installation

There is a paying SaaS option, in which case your Odoo instance is hosted by Odoo and available at https://ainulindale.odoo.com/ for instance.

Databases

You can export and duplicate databases directly from the web interface (Top Right Menu -> My Databases).

You can upload attachments (PDF or picture) to an Odoo vendor bill. There is an attachment icon next to the "Send message" section button.
When you create manually a vendor bill, you need to enter invoice lines. Don't forget to remove the VAT if it's not applicable.
If the vendor bill is not in the natural currency, you must enter the corresponding rate on the concerned date. This can be done via Accounting -> Configuration -> Currencies. Alternatively, enter directly the bill in the natural (paid) currency.

The general ledger is is accessible via Reporting -> General Ledger and is a good way to check current balances in your different accounts.
Reporting -> Profit and Loss is also very useful.

It seems the Payroll module does not have built-in rules for countries. For instance, you must enter manually the rules that will compute your social contributions.
Rules should be either of category "Company contribution" (if the employer pays this contribution) or "Deduction" (if it's paid by the employee, out of the gross salary).
You must first create a work contract for an employee.
Clicking on the "Compute Sheet" button will create automatically the salary computations (such as social compensations, etc).
To create the entries in the accounting journals, you should click on the "Create Draft Entry" button on the Payslip. Then those entries must be posted using the Accounting module.

I set all the Deduction rules (the social contributions paid by the employee) with 5099 (Net Salary) as the debit account and 2270 (Social insurances owed) as the credit account. It generates correct entries but another manual entry is absolutely mandatory, with 5099 as debit and 2260 (Salaries) as credit. I am not sure how to perform this in a better (and automated) way, but currently it's working fine.

1024 is the bank account for Revolut, 1025 is the bank account for PostFinance.
2260 holds the debt of the company to the employees (salaries).
2270 holds the debt of the company to the social insurance institutions.
2800 is for the share capital.
2850 (Private share account) is used to hold debts of the company to the shareholder.
5700, 5710, 5720, 5730, 5740 are used for various social benefits (health insurance, unemployment insurance, maternity insurance etc).
6000 for rental expenses.
6510 for legal fees (registration of company, etc).
6520 for fiduciary fees.
6700 for general operating expenses.
6900 for financial expenses like bank fees.