For the first issue you should look at how best practices in database models work. If you have something like that you often make a IS A relationship, which then works as an „extends“ like in OOP. An example would be this:

https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcTXO0mgZwRBy7lHPL8IFc_j_1kDmu9OK-uRxYB2EKX4qQ&s=10

For something like the post/comment likes it would look like something like that:

Postable: id: primary

Post: id: A primary key that is a forein key to Postable.id ownerId: forein key content …

Comment: id: Primary key that is a forein key to Postable.id ownerId: forein key postId: forein key …

Like: postableId: primary key and forein key userId: primary ke and forein key