Where can I see all my likes (reactions)? Is it possible to check their profiles and like them back?

You can see the list of the people who liked you (but whom you did not like back yet) in the 'Likes' tab of the Activity section. Their pictures might be blurred. When you purchase a subscription, you will receive a daily pack of unblurs so that you can see those profiles. 

If you see the screen with an offer to acquire unblurs, that means that you have used all of your complimentary unblurs within the subscription package. Do not worry; you can always purchase extras in the My Profile section. 

To check the profile of the user who reacted to you, please do the following:

• Open “Activity” (the heart icon at the bottom of the screen);
• Choose “Likes”;
• Choose the user you want to check;
• If the user is blurred, unblur them first;
• Tap on the photo of the unblurred user;
• Scroll up to check the account.

To like them back (react to them), tap the heart button. If you don't have any unblurs left, you can keep looking through the cards in Finder to get a match with the reactions you got. It could be hard sometimes, as it's possible your filters don't fit the accounts from the Likes list.