What happens if at 55 I put aside ERS amount but just before turning 65 I migrate, will I get the interest earned or this interest is thrown into the CPF life pool?
Nowadays nothing whatsoever is "thrown into the CPF LIFE pool" until very shortly before you start CPF LIFE payouts. If you do nothing, CPF LIFE payouts don't start until age 70. So at age 65 nothing at all happens, by default, unless you take specific voluntary action.
As long as you're alive, interest is always paid on all dollars in CPF that are on account for whole calendar months, including all dollars in the CPF Lifelong Income Fund. (You get a specific life annuity claim on your portion of dollars that end up in the CPF Lifelong Income fund, including interest on those dollars.)
Leaving Singapore is not exactly equivalent to leaving CPF or even avoiding CPF LIFE. It's not enough to "migrate." In particular, if you're a citizen of Singapore you have to terminate your citizenship. See CPF Form CPF-LM for reference.
Let's suppose you're a citizen of Singapore and you move to Belgium at age 63 with your German spouse to retire in Belgium. (Belgium and Germany are both EU countries, so this is allowed.) At age 67 you naturalize as a Belgian citizen and terminate your Singaporean citizenship. Let's assume for simplicity you haven't done anything with respect to your CPF savings, and so CPF LIFE payouts haven't started (the default). You then have two choices:
1. Leave your CPF balances intact as long as you wish -- for the rest of your life if you wish -- and they'll continue to earn interest. Please note that taxes in your new country of residence typically apply to your worldwide income, including CPF interest.
2. Make a full withdrawal any time you wish, with accrued interest for all whole calendar months.
You are not allowed to start CPF LIFE payouts if you're no longer an eligible CPF member, such as in this situation. However, if you started CPF LIFE payouts at age 66 (as a modification to the above example) and then subsequently end (or lose) your Singaporean citizenship at age 67 as above, you have the choice to continue monthly payouts or to take a single "surrender" payout.
I do not recommend you go out of your way to avoid CPF LIFE. In fact, I generally recommend you take the life annuity if you're in a position to do so. It's never a too lavish amount, it's a genuinely great value, and it's really quite wonderful as a financial backstop, especially if you can arrange to have a diversity of high quality sovereign life annuities in your retirement.