European elections take place every 5 years (the last ones in 2024). Which EU institution(s) is/are renewed after European elections?

There are about 38M inhabitants in Poland and 19M in Romania (Poland's population is about twice Romania's population). According to the principle of degressive proportionality, in collective decisions:

All the member States of the EU are affected by the Covid-19 pandemic. Various measures to tackle the health crisis have been (or could be) taken at different levels: e.g. the possibility to emit "Coronabonds" at the EU level, lockdown measures at the national level (e.g. France, Italy, but not Sweden in March/April 2020), and different restrictions at the level of cities (e.g. mask wearing, additional curfew measures).

What is the principle determining which is the adequate "level" to take such decisions?

In the absence of productivity gains, and when labour is perfectly mobile, migration of workers between country L (with low level of wages) to country H (with high level of wages)

A producer of shoes in country H considers the possibility to outsource its production in country L. While a worker from country H produces 10 shoes per day and that a worker from country L produces 5 shoes per day, the gross salary per worker in country H is 150 euros per day, and only 90 euros per day in country L.

Suppose there is a migration from country P to country R. We assume for convenience that there is a single labour market (i.e. there is no distinction between skilled and unskilled workers). Other things being equal:

Suppose that two countries A and B start to trade together. Country A specialises in the production of good X and country B specialises in the production of good Y (meaning A exports good X, while B exports good Y).

Everything else being equal (in particular, there is no gain in productivity and no labour mobility), we can observe in country B:

In the absence of productivity gains, and when labour is perfectly mobile, migration of workers from country L (with low level of wages) to country H (with high level of wages)