how the cross appeared on Frenchie's neck?

During Sister Irene's and Frenchie's confrontation with Valak, it tries to kill Frenchie. That's when Irene comes to save him and Valak throws her away. Somewhere between this Valak possessed him.

Here is what Ed says during the end,

Take Maurice Theriault. His friends called him Frenchie. He was a French-Canadian farmer. Nothing more than a third grade education. Yet, after he was possessed...

After that, this scene appears.

(Ed continues) ...spoke some of the best Latin I'd ever heard.

 

Lauren: And like that, an upside-down cross started to appear from within his body.

So, he got possessed during that confrontation and that's why an inverted cross appeared on his neck.

I would also like to know what it means and what it can be there for.

This simply means that Frenchie is in demonic possession as inverted cross symbolizes demon according to all movies of The Conjuring universe.

It also points that the Frenchie is the guy who is shown in the beginning of The Conjuring, thus connecting the events of this movie to The Nun.

From Collider,

A quick shot reveals an upside-down cross—basically Valak’s house sigil—embedded in Maurice’s neck. Flash-forward 20 years to…a scene from The Conjuring. Ed (Patrick Wilson) and Lorraine Warren deliver a lecture on possession at Massachusetts Western University while Carolyn Perron (Lili Taylor) looks on from the audience.

how the cross appeared on Frenchie's neck?

During Sister Irene's and Frenchie's confrontation with Valak, it tries to kill Frenchie. That's when Irene comes to save him and Valak throws her away. Somewhere between this Valak possessed him.

Here is what Ed says during the end,

Take Maurice Theriault. His friends called him Frenchie. He was a French-Canadian farmer. Nothing more than a third grade education. Yet, after he was possessed...

After that, this scene appears.

(Ed continues) ...spoke some of the best Latin I'd ever heard.

Lauren: And like that, an upside-down cross started to appear from within his body.

So, he got possessed during that confrontation and that's why an inverted cross appeared on his neck.

I would also like to know what it means and what it can be there for.

This simply means that Frenchie is in demonic possession as inverted cross symbolizes demon according to all movies of The Conjuring universe.

It also points that the Frenchie is the guy who is shown in the beginning of The Conjuring, thus connecting the events of this movie to The Nun.

From Collider,

A quick shot reveals an upside-down cross—basically Valak’s house sigil—embedded in Maurice’s neck. Flash-forward 20 years to…a scene from The Conjuring. Ed (Patrick Wilson) and Lorraine Warren deliver a lecture on possession at Massachusetts Western University while Carolyn Perron (Lili Taylor) looks on from the audience.

how the cross appeared on Frenchie's neck.

During Sister Irene's and Frenchie's confrontation with Valak, it tries to kill Frenchie. That's when Irene comes to save him and Valak throws her away. It's possible somewhere between this Valak possessed him.

I would also like to know what it means and what it can be there for.

This simply means that Frenchie is in demonic possession as inverted cross symbolizes demon according to all movies of The Conjuring universe.

It also points that the Frenchie is the guy who is shown in the beginning of The Conjuring, thus connecting the events of this movie to The Nun.

From Collider,

A quick shot reveals an upside-down cross—basically Valak’s house sigil—embedded in Maurice’s neck. Flash-forward 20 years to…a scene from The Conjuring. Ed (Patrick Wilson) and Lorraine Warren deliver a lecture on possession at Massachusetts Western University while Carolyn Perron (Lili Taylor) looks on from the audience.

how the cross appeared on Frenchie's neck?

During Sister Irene's and Frenchie's confrontation with Valak, it tries to kill Frenchie. That's when Irene comes to save him and Valak throws her away. Somewhere between this Valak possessed him.

Here is what Ed says during the end,

Take Maurice Theriault. His friends called him Frenchie. He was a French-Canadian farmer. Nothing more than a third grade education. Yet, after he was possessed...

After that, this scene appears.

(Ed continues) ...spoke some of the best Latin I'd ever heard.

Lauren: And like that, an upside-down cross started to appear from within his body.

So, he got possessed during that confrontation and that's why an inverted cross appeared on his neck.

I would also like to know what it means and what it can be there for.

This simply means that Frenchie is in demonic possession as inverted cross symbolizes demon according to all movies of The Conjuring universe.

It also points that the Frenchie is the guy who is shown in the beginning of The Conjuring, thus connecting the events of this movie to The Nun.

From Collider,

A quick shot reveals an upside-down cross—basically Valak’s house sigil—embedded in Maurice’s neck. Flash-forward 20 years to…a scene from The Conjuring. Ed (Patrick Wilson) and Lorraine Warren deliver a lecture on possession at Massachusetts Western University while Carolyn Perron (Lili Taylor) looks on from the audience.