Эти сандалии удобные, и ремешок у них мягкий.

Because сандалии is plural, so the demonstrative has to be plural too.

• этот = masculine singular
• эта = feminine singular
• это = neuter singular
• эти = plural

So:

• эти сандалии = these sandals

Russian words that describe a noun usually have to agree with it in number, and often also in gender and case.

In Russian, сандалии is very commonly used as a plural noun, much like English sandals.

There is a singular form, сандалия, but in everyday speech people usually talk about the footwear as a pair and use the plural:

• Эти сандалии удобные. = These sandals are comfortable.

This is similar to how some clothing items are often plural in English too, like pants or glasses.

Russian normally omits the present-tense form of to be.

So English:

• These sandals are comfortable

becomes simply:

• Эти сандалии удобные

And:

• the strap is soft

becomes:

• ремешок мягкий

Russian does use forms of быть for the past and future, but in the present tense, it is usually not said.

Удобные agrees with сандалии.

Since сандалии is plural, the adjective must also be plural:

• сандалии — plural
• удобные — plural form of удобный (comfortable)

So the phrase literally works like:

• these sandals comfortable

with the meaning These sandals are comfortable.

Yes. Both are possible:

• Эти сандалии удобные
• Эти сандалии удобны

The difference is mainly stylistic:

• удобные = long form; very common in everyday speech, sounds natural and conversational
• удобны = short form; often sounds more formal, concise, or bookish

A learner will hear the long form a lot in spoken Russian, especially in simple descriptions like this.

Here у них means something like on them, with them, or belonging to them.

So:

• ремешок у них мягкий

literally feels like:

• the strap on them is soft

This is a very natural Russian way to talk about a part or feature of something.

Why not их ремешок?

Because их ремешок can sound less natural here. Russian often prefers the у + genitive construction when describing a part, feature, or possession in this kind of sentence:

• У машины мощный двигатель. = The car has a powerful engine.
• У рубашки длинные рукава. = The shirt has long sleeves.
• У них мягкий ремешок. = They have a soft strap.

So у них is the idiomatic choice.

Because у requires the genitive case.

The pronoun они changes like this:

• они = they
• их / них = of them / them (genitive form)

After certain prepositions, Russian uses a form with н- at the beginning:

• у них
• к ним
• с ними
• для них

So у них is the correct form after у.

Russian can use the singular here to talk about a characteristic part in a general way.

So ремешок у них мягкий means that the strap part of the sandals is soft. It does not necessarily mean there is literally only one strap in total; it is just describing that feature.

Depending on context, plural could also be possible:

• ремешки у них мягкие

But the singular can sound natural when the speaker is focusing on the design feature as one element of the item.

Yes, у них ремешок мягкий is also possible.

Russian word order is flexible, and changing it often changes the focus rather than the basic meaning.

• Ремешок у них мягкий puts ремешок in focus first: As for the strap, it’s soft.
• У них ремешок мягкий starts with the idea of these sandals have... and then gives the detail.

Both are grammatical. The version in your sentence sounds natural and slightly highlights the strap as another good feature.

Because и is joining two separate clauses:

1. Эти сандалии удобные
2. ремешок у них мягкий

Each part has its own subject and description, so Russian normally puts a comma before и in this situation.

This is similar to English:

• These sandals are comfortable, and the strap is soft.

They describe different things:

• удобные = comfortable overall
• мягкий = soft to the touch or soft in feel

So the sentence is saying two related but different things:

• the sandals are comfortable in general
• the strap is soft, which helps explain that comfort

This kind of pairing is very common in product descriptions or everyday comments about clothes and shoes.

Yes, very much.

It sounds natural in everyday speech, but it also works well in contexts like:

• trying on shoes in a shop
• describing footwear to a friend
• writing a simple review
• talking about why a pair of sandals feels nice

The structure is useful because it first gives an overall opinion:

• Эти сандалии удобные

and then adds a specific detail:

• и ремешок у них мягкий

That is a very common Russian pattern: general impression + specific feature.