Sol kapı açık olunca insanlar sessizce içeri girebiliyor.

Yes. Sol means left, and here it works like an adjective before kapı (door), so sol kapı means the left door or the door on the left.

A small nuance:

• sol kapı = the left-hand door
• soldaki kapı = the door that is on the left

In many contexts, both can work, but soldaki kapı can sound a bit more explicitly locational.

This is a very common thing learners notice.

There is an important difference:

• açık kapı = an open door
  
  • Here açık directly describes the noun.
• kapı açık = the door is open
  
  • Here açık acts like the predicate of the sentence.

So in Sol kapı açık olunca, the idea is not just the left open door, but when the left door is open.

Olunca comes from olmak (to be / to become / to happen) plus the suffix -ınca / -ince, which often means:

• when
• once
• sometimes since / because

So açık olunca means roughly:

• when it is open
• once it is open
• in context, possibly since it is open

This suffix is a very common way to connect clauses in Turkish without using a separate word like when.

Because açık is an adjective, meaning open. Turkish often uses adjective + olmak to express states in clause-linking structures.

So:

• açık = open
• açık olmak = to be open
• açık olunca = when it is open

This is very natural in Turkish. You will see the same pattern with many adjectives:

• hazır olunca = when it is ready
• müsait olunca = when he/she is available
• yorgun olunca = when he/she is tired

Yes, and that is a useful comparison.

• açıkken = while it is open
• açık olunca = when it becomes/is open, often with a sense of once or since

In many situations they are close, but the nuance is slightly different:

• açıkken focuses on the state continuing
• olunca often sounds more like a condition or trigger

So this sentence could suggest:

• when / since the left door is open, people can enter quietly

Girebiliyor can be broken down like this:

• gir- = enter
• -ebil- = can / be able to
• -iyor = present continuous / current ongoing or general present situation

So girebiliyor means:

• can enter
• are able to enter
• can get in

The full idea is not just abstract ability, but ability made possible by the situation.

Both can be translated as can enter, but the feel is slightly different.

• girebilir = more neutral, general, or formal can / may enter
• girebiliyor = emphasizes that, under the current circumstances, entry is actually possible

In this sentence, girebiliyor fits well because the open door creates a real, practical possibility:

• The left door is open, so people are able to enter quietly.

It sounds a bit more situation-based than girebilir.

İçeri means inside / in and is very commonly used with motion verbs like girmek (to enter).

So:

• içeri girmek = to go in / enter
• dışarı çıkmak = to go out
• yukarı çıkmak = to go up

About the alternatives:

• içeriye girmek is possible, but içeri girmek is more common and natural in everyday speech.
• içine girmek means to go into its inside / into something, and usually points to a specific container or space:
  • odanın içine girmek = to go into the room
  • kutunun içine girmek = to go inside the box

So here içeri girebiliyor is the normal expression for can go in / can enter.

Sessizce comes from:

• sessiz = quiet / silent
• -ce / -ca = a suffix that often turns words into adverbs meaning in a ... way

So:

• sessiz = quiet
• sessizce = quietly / silently

This suffix is very common:

• yavaş → yavaşça = slowly
• güzel → güzelce = nicely / properly
• çocukça = childishly / childlike

In this sentence, sessizce tells us how people enter.

It can suggest both, depending on context.

Sol kapı açık olunca insanlar sessizce içeri girebiliyor can be understood as:

• When the left door is open, people can enter quietly.
• Since the left door is open, people can enter quietly.

This overlap is normal in Turkish. The suffix -ınca / -ince often connects time and cause very naturally.

So the sentence can carry both ideas:

• a time condition: whenever the door is open
• a reason: because it is open, entry is possible

Because Turkish normally puts the main verb at the end of the clause.

So the structure is roughly:

• Sol kapı açık olunca = when the left door is open
• insanlar sessizce içeri girebiliyor = people can quietly go in

This final-verb pattern is one of the most basic word-order features of Turkish.

The sentence could be rearranged a bit for emphasis, but the verb usually stays last:

• İnsanlar sol kapı açık olunca sessizce içeri girebiliyor.

Usually here it means people in a general sense.

Turkish does not have articles like a and the, so whether something is definite or general depends on context.

So insanlar can mean:

• people
• the people

In this sentence, it most naturally sounds general:

• people can enter quietly

If the speaker meant a specific known group, context would make that clear.